Buffer management in video codecs

In one example, a video decoder is configured to determine, from data associated with an open decoding refresh (ODR) picture of video data, an identifier for a previous picture of the video data, determine whether the previous picture is currently stored in a reference picture memory, and decode only data for pictures of the video data having display order values that are greater than a display order value of the ODR picture, when the previous picture is not stored in the reference picture memory. In another example, a video encoder is configured to encode an open decoding refresh (ODR) picture, determine a previously coded picture having a display order value less than a display order value of the ODR picture and having a temporal identifier value equal to zero, and signal syntax data for the ODR picture representative of an identifier of the determined previously coded picture.

TECHNICAL FIELD

This disclosure relates to video coding.

BACKGROUND

SUMMARY

In general, this disclosure describes techniques for improved management of a reference picture memory, such as a decoded picture buffer (DPB), when pictures are used for random access. In particular, the techniques of this disclosure are directed to determining whether an open decoding refresh (ODR) picture has been used as a random access point (RAP) and selectively decoding subsequent pictures, in coding order. Such techniques may improve DPB performance and storage management by discarding pictures no longer needed for reference from the DPB. In accordance with techniques of the disclosure, when a video decoder determines that a current picture to be decoded is an ODR picture, the video decoder may determine whether the ODR picture is used for random access. The video decoder may, for example, determine whether a DPB used in video decoding includes a reference picture having a particular display order value that is less than the display order of the ODR picture. The display order value may be signaled as a syntax element for the ODR picture. When the reference picture corresponding to the signaled display order value is absent from the DPB, video decoder may determine that the ODR picture is used for random access. An ODR picture may be conceptually the same as the Clean Random Access (CRA) picture in HEVC.

The video decoder may, consequently, mark the pictures currently included in the DPB as unused for reference for further decoding of subsequent pictures when the ODR picture is used for random access. In this way, the video decoder may remove unused pictures from the DPB, thereby potentially improving storage management of the DPB. The techniques of the disclosure may also enable the video decoder to decode only subsequent pictures that have display orders that are greater than the display order of the ODR used for random access. In this manner, by decoding only subsequent pictures with display orders greater than the display order of the ODR, the video decoder may skip outputting of data for pictures having display order values less than a display order value of the ODR picture. Techniques of the present disclosure may therefore improve the performance of the video decoder by refraining from decoding pictures that will not be displayed or used as reference pictures for further decoding, which may improve processing efficiency and/or reduce battery drain in a mobile device or other battery powered coding device.

In one example, a method includes determining that an open decoding refresh (ODR) picture of video data is being used as a random access point. For example, the method may include determining, from data associated with an open decoding refresh (ODR) picture of video data, an identifier for a previous picture of the video data. The method may also include determining whether the previous picture is currently stored in a reference picture memory. The method may further include decoding only data for pictures of the video data having display order values that are greater than a display order value of the ODR picture when the previous frame is not stored in the reference frame memory. The method may also include skipping output of data for pictures having display order values less than a display order value of the ODR picture and decoding order values greater than a decoding order value of the ODR picture.

In another example, an apparatus for decoding video data includes a video decoder configured to determine that an open decoding refresh (ODR) picture of video data is being used as a random access point. For example, the video decoder may be configured to determine, from data associated with an open decoding refresh (ODR) picture of video data, an identifier for a previous picture of the video data, determine whether the previous picture is currently stored in a reference picture memory, and decode only data for pictures of the video data having display order values that are greater than a display order value of the ODR picture, when the previous picture is not stored in the reference picture memory. The video decoder may also be configured to skip outputting of data for pictures having display order values less than a display order value of the ODR picture and decoding order values greater than a decoding order value of the ODR picture.

In another example, a computer program product includes a computer-readable storage medium having stored thereon instructions that, when executed, cause a processor of a device for decoding video data to determine that an open decoding refresh (ODR) picture of video data is being used as a random access point. For example, the computer-readable storage medium may include instructions that, when executed, cause the processor of the device to determine, from data associated with an open decoding refresh (ODR) picture of video data, an identifier for a previous picture of the video data, determine whether the previous picture is currently stored in a reference picture memory, and decode only data for pictures of the video data having display order values that are greater than a display order value of the ODR picture, when the previous picture is not stored in the reference picture memory. The computer-readable storage medium may also include instructions that, when executed, cause the processor of the device to skip outputting of data for pictures having display order values less than a display order value of the ODR picture and decoding order values greater than a decoding order value of the ODR picture

In another example, an apparatus for decoding video data includes means for determining that an open decoding refresh (ODR) picture of video data is being used as a random access point. For example, the apparatus may include means for determining, from data associated with an open decoding refresh (ODR) picture of video data, an identifier for a previous picture of the video data, means for determining whether the previous picture is currently stored in a reference picture memory, and means for decoding only data for pictures of the video data having display order values that are greater than a display order value of the ODR picture, when the previous picture is not stored in the reference picture memory. The apparatus may further include means for skipping output of data for pictures having display order values less than a display order value of the ODR picture and decoding order values greater than a decoding order value of the ODR picture.

In another example, a method of encoding video data includes encoding an open decoding refresh (ODR) picture, determining a previously coded picture having a display order value less than a display order value of the ODR picture and having a temporal identifier value equal to zero, and signaling syntax data for the ODR picture representative of an identifier of the determined previously coded picture to cause a video decoder to determine whether the ODR picture is being used for random access based on the identifier of the determined previously coded picture.

In another example, an apparatus for encoding video data is configured to encode an open decoding refresh (ODR) picture, determine a previously coded picture having a display order value less than a display order value of the ODR picture and having a temporal identifier value equal to zero, and signal syntax data for the ODR picture representative of an identifier of the determined previously coded picture to cause a video decoder to determine whether the ODR picture is being used for random access based on the identifier of the determined previously coded picture.

In another example, a computer program product includes a computer-readable storage medium having stored thereon instructions that, when executed, cause a processor of a device for decoding video data to encode an open decoding refresh (ODR) picture, determine a previously coded picture having a display order value less than a display order value of the ODR picture and having a temporal identifier value equal to zero, and signal syntax data for the ODR picture representative of an identifier of the determined previously coded picture to cause a video decoder to determine whether the ODR picture is being used for random access based on the identifier of the determined previously coded picture.

In another example, an apparatus for encoding video data includes means for encoding an open decoding refresh (ODR) picture, means for determining a previously coded picture having a display order value less than a display order value of the ODR picture and having a temporal identifier value equal to zero, and means for signaling syntax data for the ODR picture representative of an identifier of the determined previously coded picture to cause a video decoder to determine whether the ODR picture is being used for random access based on the identifier of the determined previously coded picture.

DETAILED DESCRIPTION

In general, this disclosure describes techniques for improved management of a reference picture memory, such as a decoding picture buffer (DPB), when a random access picture is used for random access. In some examples, the techniques of this disclosure are directed to determining whether an open decoding refresh (ODR) picture has been used for random access and selectively decoding subsequent pictures to improve DPB performance and storage management. Moreover, when the ODR picture is used for a random access, these techniques allow a decoder to determine whether subsequent pictures in the bitstream can be properly decoded. One example of random access picture is an instantaneous decoding refresh (IDR) picture. An IDR picture is an independently decodable I-picture of a closed Group of Pictures (GOP). A closed GOP is a grouping of pictures of a video representation that does not include any pictures that depend on pictures prior to the GOP in either decoding or presentation order to be correctly decodable. An open GOP is such a group of pictures in which pictures preceding the initial intra picture in output order may not be correctly decodable. For example, an open GOP may include one or more pictures that depend at least in part on content of at least one picture preceding the open GOP.

An ODR picture may correspond to a random access point included in an open GOP. In some examples, an ODR picture may be an I-picture, in which all coded pictures that follow the ODR picture both in decoding order and output order do not use inter prediction from any picture that precedes the ODR picture either in decoding order or output order; and any picture that precedes the ODR picture in decoding order also precedes the ODR picture in output order. Although the ODR picture may be used for random access, some of the pictures in the open GOP following the ODR picture in coding order may not be correctly decodable when decoding commences from the ODR picture. In some examples, an I-picture of an open GOP may be used as a random access point (RAP) for the commencement of playback of a video representation.

When a video encoder uses more complex prediction structures, e.g., hierarchical B-picture coding structures, more pictures may be coded as ODR pictures, thereby introducing potential complexities into random access operations. For example, when a user is seeking from one picture to another, a conventional video decoder is configured to expect that video data in the bitstream will be decoded sequentially. Therefore, when the bitstream is not decoded sequentially, as may happen when beginning decoding from an ODR that is used for random access, the video decoder may attempt to decode all pictures in decoding order. Conventionally, if an ODR picture is used as a random access point, there are leading pictures following the ODR picture in the decoding order which might not be correctly decoded. Those leading pictures may be not needed for decoding. However, if the bitstream is decoded as normal, the leading pictures may need to be decoded. Without knowing the decoding status, it may not be possible to know whether those pictures are to be decoded or not. When starting decoding from an ODR picture, the pictures in the reference picture memory may, conventionally, not be marked as unused for reference e.g., because some explicit or implicit reference picture marking processes associated with the leading pictures are not invoked. This may lead to the inefficient memory management. The techniques of this disclosure, however, provide information usable by the video decoder to indicate pictures that cannot be correctly decoded, as well as information for more efficiently managing the DPB, e.g., to discard reference pictures from the DPB that would otherwise be stored but not used for reference and/or skipping output of data for some pictures.

Accordingly, in some example techniques of the disclosure, the decoder may perform automatic status checking to determine whether pictures needed to decode other pictures of an open GOP are present in a DPB. For instance, a video decoder may initially receive a network abstraction layer (NAL) unit that includes an ODR picture to be decoded that is used for random access. When in fact the NAL unit is the next NAL unit to be decoded in sequential order, reference pictures on which pictures of an open GOP included in the NAL unit should be present in the DPB of the video decoder. On the other hand, when reference pictures on which the pictures of the open GOP included in the NAL unit are not present in the DPB, e.g., when a user seeks to an out-of-order temporal location of the video data, the video decoder may be configured to determine that an ODR picture of the open GOP in the NAL unit is being used for random access. In accordance with the techniques of this disclosure, the NAL unit may include information that identifies a picture from a previous GOP, that is, a GOP that precedes the current GOP in the bitstream. If the signaled picture exists in the DPB, the video decoder may determine that the ODR picture is not used a random access point.

If the reference picture does not exist in the DPB, the video decoder determines that the ODR picture is used as a random access point and activates an ODR random access status, in accordance with the techniques of this disclosure. The video decoder may, consequently, mark the current pictures included in the DPB as unused for reference during decoding of subsequent pictures when the random access status is activated. In this way, the video decoder may remove unused pictures from the DPB, thereby improving storage management of the DPB. The techniques of the disclosure may also enable the video decoder to decode only pictures following the ODR picture in decoding order that have display order values that are greater than the display order value of the ODR used for random access. Thus, in some examples, techniques of the disclosure enable a video decoder to skip outputting pictures having display order values less than the display order value of the ODR picture and decoding order values greater than the decoding order value of the ODR picture. In this way, techniques of the present disclosure may improve the performance of the video codec by refraining from attempting to decode and output pictures that cannot be correctly decoded.

FIG. 1is a block diagram illustrating an example video encoding and decoding system10that may utilize techniques for improved management of a reference picture memory, such as a decoding picture buffer (DPB), when pictures are signaled for random access. As shown inFIG. 1, system10includes a source device12that provides encoded video data to be decoded at a later time by a destination device14. In particular, source device12provides the video data to destination device14via a computer-readable medium16. Source device12and destination device14may comprise any of a wide range of devices, including desktop computers, notebook (i.e., laptop) computers, tablet computers, set-top boxes, telephone handsets such as so-called “smart” phones, so-called “smart” pads, televisions, cameras, display devices, digital media players, video gaming consoles, video streaming device, or the like. In some cases, source device12and destination device14may be equipped for wireless communication.

In the example ofFIG. 1, source device12includes video source18, video encoder20, and output interface22. Destination device14includes input interface28, video decoder30, and display device32. In accordance with this disclosure, video encoder20of source device12may be configured to apply the techniques for improved management of a decoding picture buffer (DPB) when a ODR picture is used for random access. In other examples, a source device and a destination device may include other components or arrangements. For example, source device12may receive video data from an external video source18, such as an external camera. Likewise, destination device14may interface with an external display device, rather than including an integrated display device.

The illustrated system10ofFIG. 1is merely one example. Techniques for improved management of a decoding picture buffer (DPB) when a picture is used for random access may be performed by any digital video encoding and/or decoding device. Although generally the techniques of this disclosure are performed by a video encoding device, the techniques may also be performed by a video encoder/decoder, typically referred to as a “CODEC.” Moreover, the techniques of this disclosure may also be performed by a video preprocessor. Source device12and destination device14are merely examples of such coding devices in which source device12generates coded video data for transmission to destination device14. In some examples, devices12,14may operate in a substantially symmetrical manner such that each of devices12,14include video encoding and decoding components. Hence, system10may support one-way or two-way video transmission between video devices12,14, e.g., for video streaming, video playback, video broadcasting, or video telephony.

Video source18of source device12may include a video capture device, such as a video camera, a video archive containing previously captured video, and/or a video feed interface to receive video from a video content provider. As a further alternative, video source18may generate computer graphics-based data as the source video, or a combination of live video, archived video, and computer-generated video. In some cases, if video source18is a video camera, source device12and destination device14may form so-called camera phones or video phones. As mentioned above, however, the techniques described in this disclosure may be applicable to video coding in general, and may be applied to wireless and/or wired applications. In each case, the captured, pre-captured, or computer-generated video may be encoded by video encoder20. The encoded video information may then be output by output interface22onto a computer-readable medium16.

Input interface28of destination device14receives information from computer-readable medium16. The information of computer-readable medium16may include syntax information defined by video encoder20, which is also used by video decoder30, that includes syntax elements that describe characteristics and/or processing of blocks and other coded units, e.g., GOPs. Display device32displays the decoded video data to a user, and may comprise any of a variety of display devices such as a cathode ray tube (CRT), a liquid crystal display (LCD), a plasma display, an organic light emitting diode (OLED) display, or another type of display device.

Video encoder20and video decoder30may operate according to a video coding standard, such as the High Efficiency Video Coding (HEVC) standard presently under development, and may conform to the HEVC Test Model (HM). Alternatively, video encoder20and video decoder30may operate according to other proprietary or industry standards, such as the ITU-T H.264 standard, alternatively referred to as MPEG-4, Part 10, Advanced Video Coding (AVC), or extensions of such standards. The techniques of this disclosure, however, are not limited to any particular coding standard. Other examples of video coding standards include MPEG-2 and ITU-T H.263. Although not shown inFIG. 1, in some aspects, video encoder20and video decoder30may each be integrated with an audio encoder and decoder, and may include appropriate MUX-DEMUX units, or other hardware and software, to handle encoding of both audio and video in a common data stream or separate data streams. If applicable, MUX-DEMUX units may conform to the ITU H.223 multiplexer protocol, or other protocols such as the user datagram protocol (UDP).

The ITU-T H.264/MPEG-4 (AVC) standard was formulated by the ITU-T Video Coding Experts Group (VCEG) together with the ISO/IEC Moving Picture Experts Group (MPEG) as the product of a collective partnership known as the Joint Video Team (JVT). In some aspects, the techniques described in this disclosure may be applied to devices that generally conform to the H.264 standard. The H.264 standard is described in ITU-T Recommendation H.264, Advanced Video Coding for generic audiovisual services, by the ITU-T Study Group, and dated March, 2005, which may be referred to herein as the H.264 standard or H.264 specification, or the H.264/AVC standard or specification. The Joint Video Team (JVT) continues to work on extensions to H.264/MPEG-4 AVC.

The JCT-VC is working on development of the HEVC standard. The HEVC standardization efforts are based on an evolving model of a video coding device referred to as the HEVC Test Model (HM). The HM presumes several additional capabilities of video coding devices relative to existing devices according to, e.g., ITU-T H.264/AVC. For example, whereas H.264 provides nine intra-prediction encoding modes, the HM may provide as many as thirty-three intra-prediction encoding modes.

In general, the working model of the HM describes that a video frame or picture may be divided into a sequence of treeblocks or largest coding units (LCU) that include both luma and chroma samples. Syntax data within a bitstream may define a size for the LCU, which is a largest coding unit in terms of the number of pixels. A slice includes a number of consecutive treeblocks in coding order. A video frame or picture may be partitioned into one or more slices. Each treeblock may be split into coding units (CUs) according to a quadtree. In general, a quadtree data structure includes one node per CU, with a root node corresponding to the treeblock. If a CU is split into four sub-CUs, the node corresponding to the CU includes four leaf nodes, each of which corresponds to one of the sub-CUs.

Each node of the quadtree data structure may provide syntax data for the corresponding CU. For example, a node in the quadtree may include a split flag, indicating whether the CU corresponding to the node is split into sub-CUs. Syntax elements for a CU may be defined recursively, and may depend on whether the CU is split into sub-CUs. If a CU is not split further, it is referred as a leaf-CU. In this disclosure, four sub-CUs of a leaf-CU will also be referred to as leaf-CUs even if there is no explicit splitting of the original leaf-CU. For example, if a CU at 16×16 size is not split further, the four 8×8 sub-CUs will also be referred to as leaf-CUs although the 16×16 CU was never split.

A CU has a similar purpose as a macroblock of the H.264 standard, except that a CU does not have a size distinction. For example, a treeblock may be split into four child nodes (also referred to as sub-CUs), and each child node may in turn be a parent node and be split into another four child nodes. A final, unsplit child node, referred to as a leaf node of the quadtree, comprises a coding node, also referred to as a leaf-CU. Syntax data associated with a coded bitstream may define a maximum number of times a treeblock may be split, referred to as a maximum CU depth, and may also define a minimum size of the coding nodes. Accordingly, a bitstream may also define a smallest coding unit (SCU). This disclosure uses the term “block” to refer to any of a CU, PU, or TU, in the context of HEVC, or similar data structures in the context of other standards (e.g., macroblocks and sub-blocks thereof in H.264/AVC).

A CU includes a coding node and prediction units (PUs) and transform units (TUs) associated with the coding node. A size of the CU corresponds to a size of the coding node and must be square in shape. The size of the CU may range from 8×8 pixels up to the size of the treeblock with a maximum of 64×64 pixels or greater. Each CU may contain one or more PUs and one or more TUs. Syntax data associated with a CU may describe, for example, partitioning of the CU into one or more PUs. Partitioning modes may differ between whether the CU is skip or direct mode encoded, intra-prediction mode encoded, or inter-prediction mode encoded. PUs may be partitioned to be non-square in shape. Syntax data associated with a CU may also describe, for example, partitioning of the CU into one or more TUs according to a quadtree. A TU can be square or non-square (e.g., rectangular) in shape.

The HEVC standard allows for transformations according to TUs, which may be different for different CUs. The TUs are typically sized based on the size of PUs within a given CU defined for a partitioned LCU, although this may not always be the case. The TUs are typically the same size or smaller than the PUs. In some examples, residual samples corresponding to a CU may be subdivided into smaller units using a quadtree structure known as “residual quad tree” (RQT). The leaf nodes of the RQT may be referred to as transform units (TUs). Pixel difference values associated with the TUs may be transformed to produce transform coefficients, which may be quantized.

A leaf-CU may include one or more prediction units (PUs). In general, a PU represents a spatial area corresponding to all or a portion of the corresponding CU, and may include data for retrieving a reference sample for the PU. Moreover, a PU includes data related to prediction. For example, when the PU is intra-mode encoded, data for the PU may be included in a residual quadtree (RQT), which may include data describing an intra-prediction mode for a TU corresponding to the PU. As another example, when the PU is inter-mode encoded, the PU may include data defining one or more motion vectors for the PU. The data defining the motion vector for a PU may describe, for example, a horizontal component of the motion vector, a vertical component of the motion vector, a resolution for the motion vector (e.g., one-quarter pixel precision or one-eighth pixel precision), a reference picture to which the motion vector points, and/or a reference picture list (e.g., List0, List1, or List C) for the motion vector.

A leaf-CU having one or more PUs may also include one or more transform units (TUs). The transform units may be specified using an RQT (also referred to as a TU quadtree structure), as discussed above. For example, a split flag may indicate whether a leaf-CU is split into four transform units. Then, each transform unit may be split further into further sub-TUs. When a TU is not split further, it may be referred to as a leaf-TU. Generally, for intra coding, all the leaf-TUs belonging to a leaf-CU share the same intra prediction mode. That is, the same intra-prediction mode is generally applied to calculate predicted values for all TUs of a leaf-CU. For intra coding, a video encoder may calculate a residual value for each leaf-TU using the intra prediction mode, as a difference between the portion of the CU corresponding to the TU and the original block. A TU is not necessarily limited to the size of a PU. Thus, TUs may be larger or smaller than a PU. For intra coding, a PU may be collocated with a corresponding leaf-TU for the same CU. In some examples, the maximum size of a leaf-TU may correspond to the size of the corresponding leaf-CU.

Moreover, TUs of leaf-CUs may also be associated with respective quadtree data structures, referred to as residual quadtrees (RQTs). That is, a leaf-CU may include a quadtree indicating how the leaf-CU is partitioned into TUs. The root node of a TU quadtree generally corresponds to a leaf-CU, while the root node of a CU quadtree generally corresponds to a treeblock (or LCU). TUs of the RQT that are not split are referred to as leaf-TUs. In general, this disclosure uses the terms CU and TU to refer to leaf-CU and leaf-TU, respectively, unless noted otherwise.

A video sequence typically includes a series of video frames or pictures. A group of pictures (GOP) generally comprises a series of one or more of the video pictures. A GOP may include syntax data in a header of the GOP, a header of one or more of the pictures, or elsewhere, that describes a number of pictures included in the GOP. Each slice of a picture may include slice syntax data that describes an encoding mode for the respective slice. Video encoder20typically operates on video blocks within individual video slices in order to encode the video data. A video block may correspond to a coding node within a CU. The video blocks may have fixed or varying sizes, and may differ in size according to a specified coding standard.

As an example, the HM supports prediction in various PU sizes. Assuming that the size of a particular CU is 2N×2N, the HM supports intra-prediction in PU sizes of 2N×2N or N×N, and inter-prediction in symmetric PU sizes of 2N×2N, 2N×N, N×2N, or N×N. The HM also supports asymmetric partitioning for inter-prediction in PU sizes of 2N×nU, 2N×nD, nL×2N, and nR×2N. In asymmetric partitioning, one direction of a CU is not partitioned, while the other direction is partitioned into 25% and 75%. The portion of the CU corresponding to the 25% partition is indicated by an “n” followed by an indication of “Up”, “Down,” “Left,” or “Right.” Thus, for example, “2N×nU” refers to a 2N×2N CU that is partitioned horizontally with a 2N×0.5N PU on top and a 2N×1.5N PU on bottom.

Following intra-predictive or inter-predictive coding using the PUs of a CU, video encoder20may calculate residual data for the TUs of the CU. The PUs may comprise syntax data describing a method or mode of generating predictive pixel data in the spatial domain (also referred to as the pixel domain) and the TUs may comprise coefficients in the transform domain following application of a transform, e.g., a discrete cosine transform (DCT), an integer transform, a wavelet transform, or a conceptually similar transform to residual video data. The residual data may correspond to pixel differences between pixels of the unencoded picture and prediction values corresponding to the PUs. Video encoder20may form the TUs including the residual data for the CU, and then transform the TUs to produce transform coefficients for the CU.

Following any transforms to produce transform coefficients, video encoder20may perform quantization of the transform coefficients. Quantization generally refers to a process in which transform coefficients are quantized to possibly reduce the amount of data used to represent the coefficients, providing further compression. The quantization process may reduce the bit depth associated with some or all of the coefficients. For example, an n-bit value may be rounded down to an m-bit value during quantization, where n is greater than m.

Following quantization, the video encoder may scan the transform coefficients, producing a one-dimensional vector from the two-dimensional matrix including the quantized transform coefficients. The scan may be designed to place higher energy (and therefore lower frequency) coefficients at the front of the array and to place lower energy (and therefore higher frequency) coefficients at the back of the array. In some examples, video encoder20may utilize a predefined scan order to scan the quantized transform coefficients to produce a serialized vector that can be entropy encoded. In other examples, video encoder20may perform an adaptive scan. After scanning the quantized transform coefficients to form a one-dimensional vector, video encoder20may entropy encode the one-dimensional vector, e.g., according to context-adaptive variable length coding (CAVLC), context-adaptive binary arithmetic coding (CABAC), syntax-based context-adaptive binary arithmetic coding (SBAC), Probability Interval Partitioning Entropy (PIPE) coding or another entropy encoding methodology. Video encoder20may also entropy encode syntax elements associated with the encoded video data for use by video decoder30in decoding the video data.

To perform CABAC, video encoder20may assign a context within a context model to a symbol to be transmitted. The context may relate to, for example, whether neighboring values of the symbol are non-zero or not. To perform CAVLC, video encoder20may select a variable length code for a symbol to be transmitted. Codewords in VLC may be constructed such that relatively shorter codes correspond to more probable symbols, while longer codes correspond to less probable symbols. In this way, the use of VLC may achieve a bit savings over, for example, using equal-length codewords for each symbol to be transmitted. The probability determination may be based on a context assigned to the symbol.

Video encoder20may further send syntax data, such as block-based syntax data, picture-based syntax data, and GOP-based syntax data, to video decoder30, e.g., in a picture header, a block header, a slice header, or a GOP header. The GOP syntax data may describe a number of pictures in the respective GOP, and the picture syntax data may indicate an encoding/prediction mode used to encode the corresponding picture.

In accordance with techniques of the present disclosure to improve management of a reference picture memory, video encoder20, in some examples, initially receives video data from video source18. Video encoder20may partition the video data into pictures, which may be subsequently encoded by video encoder20. In some examples, video encoder20may group pictures together as a Group of Pictures (GOP). A GOP may be described as a grouping of sequential (in presentation order) pictures of a video sequence.

A display order of pictures included in a GOP may be different than a decoding order of the pictures. For example, a GOP may include any combination of I, B, or P-pictures. Intra-coded pictures (I-pictures) are those pictures that are independently decodable, meaning that to decode an I-picture, a decoder need not rely on content of other pictures. P-pictures may be inter-coded relative to one or more pictures in one direction. B-pictures may be inter-coded relative to one or more pictures in two directions. A GOP that includes only pictures that are correctly decodable without relying on content of pictures outside of the GOP may be considered a closed GOP. An open GOP, by contrast, includes pictures that depend from other pictures external to the GOP to be correctly decoded.FIGS. 4 and 5, further described herein, illustrate further details of a closed GOP102and open GOP152.

To encode video data as B or P-pictures, video encoder20may store reference pictures in a reference picture memory that B or P-pictures depend on for proper encoding. Reference pictures stored in a reference picture memory may include encoded pictures that precede a currently encoded picture in decoding order. Video encoder20, in some examples, searches the reference pictures in the reference picture memory to identify one or more suitable reference pictures that the picture currently being encoded may reference. Examples of reference picture memories are shown inFIGS. 2 and 3as reference picture memory64and82, respectively. Similar to video encoder20, video decoder30may also include a reference picture memory. Video decoder30may use the reference pictures stored in a reference picture memory of video decoder30to decode B and P-pictures. In this way, reference picture memories used by video encoder20and video decoder30may be used to encode and decode pictures of a GOP.

Source device12may further include an encapsulation unit (not shown), which may format the compressed video data for transmission into so-called “network abstraction layer units” or NAL units. Each NAL unit may include a header that identifies a type of data stored to the NAL unit. There are two types of data that are commonly stored to NAL units. The first type of data stored to a NAL unit is video coding layer (VCL) data, which includes the compressed video data, such as a coded slice of video data. The second type of data stored to a NAL unit is referred to as non-VCL data, which includes additional information such as parameter sets that define header data common to a large number of NAL units and supplemental enhancement information (SEI). The encapsulation unit may send formed NAL units to destination device14via output interface22.

Input interface28may receive the NAL units, which may pass the NAL units to a decapsulation unit (not shown). In some examples, the decapsulation unit decapsulates encoded video from NAL units in the order that the NAL units are received. Following decapsulation, video decoder30may receive the decapsulated video data from the decapsulation unit. Video decoder30then decodes the video data for display at display device32. For instance, when video decoder30decodes pictures for display in sequential display order, pictures included in the video data are decoded and the decoded pictures are then stored in a reference picture memory of video decoder30. The decoded pictures may then be used as reference pictures for subsequently decoded pictures. Because the reference picture memory may have finite storage space, video decoder30may mark pictures as unused for reference when no longer needed to decode pictures. Video decoder30may, consequently, remove unused reference pictures from the reference picture memory when no longer needed. In this way, pictures of a video sequence are decoded and displayed by video decoder30and display device32.

In general, video decoders are configured to decode an entire bitstream following an instantaneous decoding refresh (IDR) picture. However, video decoder30may be configured to begin decoding a sequence of video data starting at an open decoding refresh (ODR) picture. This may occur in response to random access, e.g., a seek to a particular temporal location of the video sequence by a user. A randomly accessed picture may generally correspond to a picture that does not immediately follow the previously decoded picture in decoding order within the bitstream.

To illustrate an example of random access, a user may request to view a video that is displayed by display device32. An application, such as a web browser or a media player, executing on destination device14may enable the user to control the display of the video. The video may be displayed based on pictures that are decoded by video decoder30. In the current example, a user may provide input to randomly access a different location of the video, such that the input is received by the application. The application may identify a picture associated with the selected location and request the picture, e.g., from source device12during network streaming or from a storage medium accessible by input interface28.

In response to receiving the identifier of the picture to select a new temporal location in the video of the current example, the application may identify a RAP picture that corresponds to the selected location of the video. In some examples, the RAP picture may be the closest ODR or IDR picture to the requested location of the video that precedes the picture of requested location in decoding order. Examples of an independently decodable pictures that video decoder30may use to provide random access include instantaneous decoding refresh (IDR) and open decoding refresh (ODR) RAP pictures. As previously described, an IDR picture is an independently decodable I-picture of a closed GOP, while an ODR picture is an I-picture of an open GOP.

The application may request encoded video data that begins with the RAP picture. In some examples, the request includes an identifier of the RAP picture. In response to receiving the request, source device12may use the identifier of the RAP picture begin sending a stream of encoded video data starting at the RAP picture to destination device14. Video decoder30may subsequently begin decoding the stream of encoded video for display by display device32.

In the example of starting with an IDR RAP picture, video decoder30may determine that the type of the RAP picture is an IDR RAP picture. Because, as previously described, an IDR RAP picture is included in a closed GOP, other pictures included in the closed GOP do not depend on reference pictures external to the GOP to be correctly decoded. Video decoder30may therefore mark all reference pictures currently stored in the reference picture memory as unused in response to determining that the RAP picture is an IDR RAP picture. Consequently, video decoder30may remove the unused reference pictures from the reference picture memory. In this way, video decoder30frees space in the reference picture memory to store the decoded IDR picture of the closed GOP and other subsequently decoded pictures included in the closed GOP.

In accordance with techniques of the present disclosure, random access may be extended to Open Decoding Refresh (ODR) pictures. Stated another way, an ODR picture may also be used for random access when video decoder30performs random access. An ODR picture is an I-picture of an open GOP that may be used for random access for the commencement of playback of a video representation.

When relatively complicated prediction structures are used, e.g., hierarchical B-picture coding structures, video encoder20may code more pictures as ODR pictures, thereby introducing complexities when a conventional client device performs random access. For example, when a user provides input to seek to a particular temporal location, a conventional video decoder is not prepared stop sequential decoding and decode from a selected random access point which is configured to expect that video data in the bitstream will be decoded sequentially. Therefore, when the bitstream is not decoded sequentially, as may happen when beginning decoding from an ODR RAP, the video decoder may attempt to decode all pictures in decoding order. The techniques of this disclosure, however, provide information usable by the video decoder to determine pictures that cannot be correctly decoded, as well as information for more efficiently managing the DPB, e.g., to discard reference pictures from the DPB that would otherwise be stored but not used for reference.

Techniques of the present disclosure may improve memory control of the reference picture memory of video decoder30by performing automatic status checking to determine whether an ODR picture has been selected for random access. In some example techniques, video encoder20may signal an identifier of a previous picture in a slice header of a slice of an ODR picture that video decoder30may use to determine whether the ODR picture has been selected for random access. If video decoder30determines that the previous picture signaled in the slice header is presently stored in the reference picture memory, video decoder30may determine that the ODR picture is not being used for random access, and decode the bitstream normally. On the other hand, if the previous picture signaled in the slice header is not presently stored in the reference picture memory, video decoder30may determine that the ODR picture is being used for random access. In response to determining that the ODR picture is being used for random access, video decoder30may mark pictures in the reference picture memory as unused, and in some examples, refrain from decoding pictures that are not required to perform further decoding of pictures that follow the ODR RAP in display order, e.g., pictures that cannot be properly decoded.

Initially, video encoder20may encode an ODR picture in a video sequence. During the encoding process, video encoder20may determine a particular previously encoded picture (or “previous picture”) having a display order that is less than the display order of the ODR picture, and that is used to indicate whether the ODR picture is being used for random access. In some examples, one or more pictures of the open GOP (herein “leading pictures”) that includes the encoded ODR picture may depend on the determined previous picture for proper decoding. Leading pictures may have a display order that is less than the ODR picture but a decoding order that is greater than the ODR picture. Video decoder20may determine the previous picture by determining, based all or in part on, whether one or more leading pictures of the open GOP depend on the previous picture. In some examples, the video encoder20may determine as the particular previously encoded picture, a picture that is the closest picture, in decoding and display order, prior to the ODR picture and that has a temporal level equal to 0.

When video encoder20determines the particular previous picture, video encoder20may signal an identifier of the previous picture in a slice header of a slice of the ODR picture (or each slice of the ODR picture). In some examples, the identifier may be a Picture Order Count (POC) value that indicates the display order of the previous picture. The POC value of the previous picture may be stored in the slice header or picture parameter set of the ODR picture. In one example, the slice header may include a syntax element identified by the name “pre_pic_POC.” The pre_pic_POC value may specify the POC value of the particular previous picture determined by video encoder20. Table 1 below indicates an example slice header of an ODR picture that includes a pre_pic_POC value, in accordance with aspects of the present disclosure. As can be seen in Table 1, the video encoder20sets the OdrPicFlag syntax element to true when the NAL unit type indicates that the slice is an ODR NAL unit or a flag in the NAL unit header indicates the slice is an ODR NAL unit.

While a POC value of the previous picture was used to illustrate one example of an identifier of the previous picture, other values are also suitable. For instance, video encoder20may signal a value equal to the difference between the POC value of the previous picture and the POC value of the ODR picture in a slice header of a slice of the ODR picture. In another example, a decoding order value (e.g., a syntax element “picture_num”) of the previous picture may be stored by video encoder20in the header of the slice. In still other examples, video encoder20may signal a value equal to the difference between the decoding order value of the previous picture and the decoding order value of the ODR picture in the header of the slice. In this manner, video encoder20may signal an identifier of the particular previous picture for an ODR picture, to indicate whether the ODR picture is being used for random access based on whether the particular previous picture is currently in the decoded picture buffer.

In the current example, after signaling the POC value of the previous picture in the slice header, an encapsulation unit may further encapsulate the ODR picture in a NAL unit (and/or encapsulate a picture parameter set signaling the identifier for the particular previous picture). Output interface22may subsequently send the NAL unit to destination device14along with other NAL units that include encoded video data. Input interface28may receive the NAL units and send the NAL units to video decoder30. In some examples, a decapsulation unit may decapsulate encoded video data from the NAL units prior to decoding the encoded video data.

In one example, a user may provide input at destination device14to select a different temporal location of a currently playing video, or to select an initial starting point for a newly requested video that is other than the very beginning of the video. An ODR picture may be the closest picture to the randomly selected location and may therefore be used by video decoder30for random access. It should be understood that absent the signaled value for the ODR picture, video decoder30would not be capable of determining that the ODR picture is being used for random access. By including the signaled value of the particular previous picture for the ODR picture, however, the techniques of this disclosure allow video decoder30to determine whether the ODR picture is being used for random access. In the current example, because the user has selected a different temporal location of the currently playing video, video decoder30may receive encoded video data that begins with the ODR RAP picture. Video decoder30may subsequently begin decoding the stream of encoded video for display by display device32.

In accordance with techniques of the present disclosure, video decoder30may begin decoding a bitstream at a particular picture. Video decoder30may determine whether a current picture is an ODR picture. For instance, video decoder30may determine whether a syntax element in a slice header of a slice in the picture indicates that the picture is an ODR picture. As one example, the OdrPicFlag variable as shown in Table 1 may be set to a value of true in the slice header of a slice in a picture to indicate the picture is an ODR picture.

Moreover, video decoder30may determine a value for a signaled identifier of a particular previous picture for the ODR picture, e.g., signaled in a slice header of a slice of the ODR picture or in a PPS corresponding to the ODR picture. For instance, video decoder30may identify a syntax element in the slice header of the slice in the ODR picture that identifies the particular previous picture. As described above, the ODR picture may include a pre_pic_POC syntax element, which indicates a display order value of the particular previous picture. Video decoder30may then determine whether the particular previous picture is stored in a reference picture memory of video decoder30by comparing the value of the pre_pic_POC syntax element with display order values (e.g., POC values) of pictures stored in the reference picture memory to identify a match.

If the previous picture is stored in the reference picture memory, video decoder30decodes pictures of the bitstream in a conventional manner. Consequently, video decoder30may maintain a status indicating that video decoder30is decoding pictures to provide normal, sequential display of pictures. In one example, a variable having a name “random_access” that maintains a value indicative of a random access status may be set to false when video decoder30determines that the previous picture is presently stored in the reference picture memory. Consequently, video decoder30may continue decoding subsequent pictures for normal, sequential display.

On the other hand, video decoder30may determine that the reference picture memory does not include the particular previous picture identified by the identifier associated with the ODR picture (e.g., the pre_pic_POC syntax element). In such examples, the user may have selected a different temporal location of a video. Consequently, video decoder30may determine that the ODR picture is being used for random access, based on the determination that the particular previous picture identified by the signaled value is not stored in the reference picture memory. Video decoder30may therefore set the random_access variable to true because video decoder30has determined that the ODR picture is being used for random access.

In the current example, video decoder30, in response to determining an ODR picture is being used for random access, may mark pictures in the reference picture memory as unused for reference after decoding the ODR picture (other than the ODR picture itself). The random_access status may be left unchanged to the value of true after decoding the ODR picture. Video decoder30may subsequently delete the pictures from the reference picture memory having the unused status.

When video decoder30is performing random access (e.g., random_access is set to true), video decoder30may disregard certain memory management operations specified in slice headers of slices in video pictures, e.g., slices of the current, open GOP. For example, video decoder30may determine syntax information signaled in a slice of a picture that indicates at least one memory management process to be performed. Video decoder30may disregard the memory management process when performing random access, e.g., the random access status is activated. To illustrate, when not in random access mode, video decoder30may be configured to apply memory management techniques to mark decoded pictures in a reference picture memory as unused for reference in response to data specified in certain slice headers. Such techniques may include sliding window techniques or explicit marking techniques. However, when in random access mode, as determined by the techniques of this disclosure, video decoder30may not have actually decoded these pictures, and therefore, these pictures may not exist in the decoded picture buffer of the reference picture memory. Consequently, in accordance with techniques of the disclosure, when video decoder30determines it is performing random access, video decoder30may disregard such memory management operations specified in a slice header because reference pictures that are to be marked as unused may not exist in the reference picture memory. In this way, video decoder30may refrain from attempting to perform management operations on reference pictures that do not exist in the reference picture memory.

Video decoder30may also, in response to determining that the ODR picture is being used for random access, decode only data for pictures of the video data having display order values that are greater than a display order value of the ODR picture. For instance, when an ODR picture is being used for random access (e.g., random_access is set to true), video decoder30may further create a variable to store the POC value of the ODR RAP picture. In one example, the variable may be named “CurrODRPOC.” Video decoder30, upon determining that an ODR picture is being used for random access, may set CurrODRPOC to the POC value of the ODR picture. In some examples, video decoder30may, based on the determination that the ODR picture is being used as a random access point, skip outputting of data for pictures having display order values less than a display order value of the ODR picture and decoding order values greater than a decoding order value of the ODR picture.

As video decoder30receives subsequent NAL units from source device12, video decoder30may parse the slice headers of slices included in each NAL unit. For each NAL unit, video decoder30may determine whether the POC value of the slice included in the NAL unit. For instance, in one example, video decoder30may compare the POC value of a slice included in a NAL unit to the POC value stored in CurrODRPOC. If the POC value of the slice is smaller than the POC value of the ODR RAP picture, video decoder30may skip decoding the encoded video data in the NAL unit. In this way, video decoder30may refrain from decoding video data of NAL units that are not required to decode from the selected ODR RAP picture. Video decoder30may also therefore skip outputting of such decoding video data of the NAL units. Consequently, techniques of the present disclosure may reduce unnecessary decoding of video data when video decoder30determines that an ODR RAP picture has been selected for random access. While the previous techniques parsed POC values of slice headers, techniques of the present disclosure may similarly parse syntax elements of, for example, a picture parameter set, which may include a POC value of a picture.

Video decoder30, in some examples, may determine that the POC value of the slice in a NAL unit is larger than the POC value of the ODR RAP picture. In such examples, video decoder30may decode the slice. In response to determining that the POC value of the slice in a NAL unit is larger than the POC value of the ODR RAP picture, video decoder30may also change the random access status to indicate decoding of pictures for normal, sequential display of pictures rather than random access. For example, video decoder30may deactivate the random access status by setting random_access to false. In this way, video decoder30may change the random access status to indicate decoding of pictures for normal, sequential display of pictures rather than random access.

In some examples, one or more components that are separate from video decoder30and/or video encoder20may perform techniques of the present disclosure to detect random access. For instance, streaming systems based on Hypertext Transfer Protocol (HTTP) or local playback applications may include components that are separate from video decoder30. In one example, a preprocessor component that is separate from video decoder30may receiving data for a request to begin playback from a particular temporal instance of the video data. The preprocessor component may identify the temporal instance based on a timestamp. If the preprocessor component determines that the closest RAP picture to the new temporal location is an ODR picture, the preprocessor component may determine that the display order of the ODR picture corresponds to the selected temporal instance of the video data. The timestamp may be a value that indicates a time at which the ODR picture is displayed in a video sequence. In response to determining that the ODR picture is used for random access, the preprocessor component may select information that indicates the timestamp of the ODR picture and send the information to video decoder30. Video decoder30may consequently use the information to perform random access using techniques of the present disclosure.

Video encoder20and video decoder30each may be implemented as any of a variety of suitable encoder or decoder circuitry, as applicable, such as one or more microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), discrete logic circuitry, software, hardware, firmware or any combinations thereof. Each of video encoder20and video decoder30may be included in one or more encoders or decoders, either of which may be integrated as part of a combined video encoder/decoder (CODEC). A device including video encoder20and/or video decoder30may comprise an integrated circuit, a microprocessor, and/or a wireless communication device, such as a cellular telephone.

FIG. 2is a block diagram illustrating an example of video encoder20that may implement techniques for improved management of a reference picture memory, such as a decoding picture buffer (DPB), when pictures are signaled for random access. Video encoder20may perform intra- and inter-coding of video blocks within video slices. Intra-coding relies on spatial prediction to reduce or remove spatial redundancy in video within a given video frame or picture. Inter-coding relies on temporal prediction to reduce or remove temporal redundancy in video within adjacent frames or pictures of a video sequence. Intra-mode (I mode) may refer to any of several spatial based prediction modes. Inter-modes, such as uni-directional prediction (P mode) or bi-prediction (B mode), may refer to any of several temporal-based prediction modes.

As shown inFIG. 2, video encoder20receives a current video block within a video picture to be encoded. In the example ofFIG. 2, video encoder20includes mode select unit40, reference picture memory64, encoding control unit66, summer50, transform processing unit52, quantization unit54, and entropy coding unit56. In some examples, reference picture memory64may be referred to in this disclosure as a picture buffer or decoding picture buffer. Mode select unit40, in turn, includes motion compensation unit44, motion estimation unit42, intra-prediction unit46, and partition unit48. For video block reconstruction, video encoder20also includes inverse quantization unit58, inverse transform unit60, and summer62. A deblocking filter (not shown inFIG. 2) may also be included to filter block boundaries to remove blockiness artifacts from reconstructed video. If desired, the deblocking filter would typically filter the output of summer62. Additional filters (in loop or post loop) may also be used in addition to the deblocking filter. Such filters are not shown for brevity, but if desired, may filter the output of summer50(as an in-loop filter).

During the encoding process, video encoder20receives a video picture or slice to be coded. The picture or slice may be divided into multiple video blocks. Motion estimation unit42and motion compensation unit44perform inter-predictive coding of the received video block relative to one or more blocks in one or more reference pictures to provide temporal prediction. Intra-prediction unit46may alternatively perform intra-predictive coding of the received video block relative to one or more neighboring blocks in the same picture or slice as the block to be coded to provide spatial prediction. Video encoder20may perform multiple coding passes, e.g., to select an appropriate coding mode for each block of video data.

Moreover, partition unit48may partition blocks of video data into sub-blocks, based on evaluation of previous partitioning schemes in previous coding passes. For example, partition unit48may initially partition a picture or slice into LCUs, and partition each of the LCUs into sub-CUs based on rate-distortion analysis (e.g., rate-distortion optimization). Mode select unit40may further produce a quadtree data structure indicative of partitioning of an LCU into sub-CUs. Leaf-node CUs of the quadtree may include one or more PUs and one or more TUs.

Mode select unit40may select one of the coding modes, intra or inter, e.g., based on error results, and provides the resulting intra- or inter-coded block to summer50to generate residual block data and to summer62to reconstruct the encoded block for use as a reference picture. Mode select unit40also provides syntax elements, such as motion vectors, intra-mode indicators, partition information, and other such syntax information, to entropy coding unit56.

Motion estimation unit42and motion compensation unit44may be highly integrated, but are illustrated separately for conceptual purposes. Motion estimation, performed by motion estimation unit42, is the process of generating motion vectors, which estimate motion for video blocks. A motion vector, for example, may indicate the displacement of a PU of a video block within a current video frame or picture relative to a predictive block within a reference picture (or other coded unit) relative to the current block being coded within the current picture (or other coded unit). A predictive block is a block that is found to closely match the block to be coded, in terms of pixel difference, which may be determined by sum of absolute difference (SAD), sum of square difference (SSD), or other difference metrics. In some examples, video encoder20may calculate values for sub-integer pixel positions of reference pictures stored in reference picture memory64. For example, video encoder20may interpolate values of one-quarter pixel positions, one-eighth pixel positions, or other fractional pixel positions of the reference picture. Therefore, motion estimation unit42may perform a motion search relative to the full pixel positions and fractional pixel positions and output a motion vector with fractional pixel precision.

Motion compensation, performed by motion compensation unit44, may involve fetching or generating the predictive block based on the motion vector determined by motion estimation unit42. Again, motion estimation unit42and motion compensation unit44may be functionally integrated, in some examples. Upon receiving the motion vector for the PU of the current video block, motion compensation unit44may locate the predictive block to which the motion vector points in one of the reference picture lists. Summer50forms a residual video block by subtracting pixel values of the predictive block from the pixel values of the current video block being coded, forming pixel difference values, as discussed below. In general, motion estimation unit42performs motion estimation relative to luma components, and motion compensation unit44uses motion vectors calculated based on the luma components for both chroma components and luma components. Mode select unit40may also generate syntax elements associated with the video blocks and the video slice for use by video decoder30in decoding the video blocks of the video slice.

Intra-prediction unit46may intra-predict a current block, as an alternative to the inter-prediction performed by motion estimation unit42and motion compensation unit44, as described above. In particular, intra-prediction unit46may determine an intra-prediction mode to use to encode a current block. In some examples, intra-prediction unit46may encode a current block using various intra-prediction modes, e.g., during separate encoding passes, and intra-prediction unit46(or mode select unit40, in some examples) may select an appropriate intra-prediction mode to use from the tested modes.

For example, intra-prediction unit46may calculate rate-distortion values using a rate-distortion analysis for the various tested intra-prediction modes, and select the intra-prediction mode having the best rate-distortion characteristics among the tested modes. Rate-distortion analysis generally determines an amount of distortion (or error) between an encoded block and an original, unencoded block that was encoded to produce the encoded block, as well as a bitrate (that is, a number of bits) used to produce the encoded block. Intra-prediction unit46may calculate ratios from the distortions and rates for the various encoded blocks to determine which intra-prediction mode exhibits the best rate-distortion value for the block.

Video encoder20forms a residual video block by subtracting the prediction data from mode select unit40from the original video block being coded. Summer50represents the component or components that perform this subtraction operation. Transform processing unit52applies a transform, such as a discrete cosine transform (DCT) or a conceptually similar transform, to the residual block, producing a video block comprising residual transform coefficient values. Transform processing unit52may perform other transforms which are conceptually similar to DCT. Wavelet transforms, integer transforms, sub-band transforms or other types of transforms could also be used. In any case, transform processing unit52applies the transform to the residual block, producing a block of residual transform coefficients. The transform may convert the residual information from a pixel value domain to a transform domain, such as a frequency domain. Transform processing unit52may send the resulting transform coefficients to quantization unit54. Quantization unit54quantizes the transform coefficients to further reduce bit rate. The quantization process may reduce the bit depth associated with some or all of the coefficients. The degree of quantization may be modified by adjusting a quantization parameter. In some examples, quantization unit54may then perform a scan of the matrix including the quantized transform coefficients. Alternatively, entropy encoding unit56may perform the scan.

Following quantization, entropy coding unit56entropy codes the quantized transform coefficients. For example, entropy coding unit56may perform context adaptive variable length coding (CAVLC), context adaptive binary arithmetic coding (CABAC), syntax-based context-adaptive binary arithmetic coding (SBAC), probability interval partitioning entropy (PIPE) coding or another entropy coding technique. In the case of context-based entropy coding, context may be based on neighboring blocks. Following the entropy coding by entropy coding unit56, the encoded bitstream may be transmitted to another device (e.g., video decoder30) or archived for later transmission or retrieval.

Video encoder20may send the encoded video data to an encapsulation unit. The encapsulation unit may format the encoded video data for transmission in the form of NAL units. Each NAL unit may include a header that identifies a type of data stored to the NAL unit. There are two types of data that are commonly stored to NAL units. The encapsulation unit may send the NAL units to an output interface which may send the NAL units to a destination device.

Video encoder20is now further described with respect to techniques of the present disclosure. Initially, video encoder20may encode an ODR picture in a video sequence. When video encoder20encodes an ODR picture, encoding control unit66may determine a previously encoded picture that has a display order that is less than the display order of the ODR picture. In some examples, encoding control unit66may search reference picture memory64to identify the previously encoded picture. Encoding control unit66may determine the previous picture by determining whether one or more pictures of an open GOP depend on the previous picture. For instance, one or more pictures of the open GOP (e.g., leading pictures) that includes the encoded ODR picture may depend on the previous picture for proper decoding. In some examples, encoding control unit66may determine an identifier for a previously coded picture that has a temporal layer equal to zero and that was the most recent previously decoded picture prior to the current ODR picture in decoding and display order.

When encoding control unit66determines the identifier for the previous picture, encoding control unit66may signal a value representative of the identifier for the previous picture in a slice header of a slice in the ODR picture. In some examples, the identifier may be a Picture Order Count (POC) value that indicates the display order of the previous picture. Encoding control unit66may store the POC value of the previous picture in a slice header of a slice of the ODR picture. In one example, the slice header may include a syntax element identified by the name “pre_pic_POC.” The pre_pic_POC value may specify the POC value of the previous picture determined by encoding control unit66.

After encoding control unit66has stored the POC value of the previous picture in the slice header of the slice in the ODR picture, the encapsulation unit may further encapsulate the ODR picture in a NAL unit. The encapsulation unit may send the NAL unit to an output interface that subsequently sends the NAL unit to a destination device along with other NAL units that include encoded video data. A destination device that includes a video decoder such as video decoder30may receive the NAL units and perform decoding of the one or more NAL units as further described inFIG. 3.

In some examples, encoding control unit66may determine the previous picture signaled in the ODR picture by comparing temporal level values of reference pictures stored in reference picture memory64to identify one or more reference pictures with a lowest temporal level value. A temporal level value may indicate decoding dependencies between one or more pictures. In one example, a lowest temporal level value may be a temporal level value of 0. Other temporal values may include numeric or alphanumeric values. Once encoding control unit66has identified one or more reference picture with a lowest temporal level, encoding control unit66may determine one of the reference pictures as the previous picture by identifying the reference picture that has a display order less than the display order of the ODR picture. In examples where encoding control unit66determines that more than one reference picture have a lowest temporal value, encoding control unit66may determine the reference picture that is closest in display order to the ODR picture and has a display order that is less than the ODR picture.

Encoding control unit66, in some examples, may determine the previous picture signaled in the ODR picture by comparing the decoding order of the ODR picture with the decoding order of reference pictures stored in reference picture memory64to identify a reference picture with a decoding order that is nearest to the decoding order of the ODR picture. For instance, encoding control unit66may search the reference pictures stored in reference picture memory64to determine a reference picture as the previous picture which has a decoding order nearest to the decoding order of the ODR picture. Encoding control unit may only search for references pictures with a display order that is less than the ODR picture. In some examples, each picture has a decoding order value which encoding control unit66may use to determine the decoding order of each respective picture. Once encoding control unit66has identified a picture that is closest to the ODR picture in decoding order with a display order that is less than the ODR picture, encoding control unit66may determine the picture as the previous picture.

Encoding control unit66, in some examples, may determine the previous picture signaled in the ODR picture by comparing the display order of the ODR picture with the display orders of reference pictures stored in reference picture memory64to identify a reference picture with a display order that is nearest to the display order of the ODR picture. For instance, encoding control unit66may search the reference pictures stored in reference picture memory64to determine a reference picture as the previous picture which has a display order nearest to the decoding order of the ODR picture. Encoding control unit66may only search for references pictures with a display order that is less than the ODR picture. Once encoding control unit66has identified a picture that is closest to the ODR picture in display order with a display order that is less than the ODR picture, encoding control unit66may determine the picture as the previous picture.

As previously described, encoding control unit66may determine a previous picture to be signaled in the ODR picture based on temporal level, decoding order, or display order. In some examples, encoding control unit66may determine a previous picture based all or in part on any combination of temporal level, decoding order, or display order to determine the previous picture. Encoding control unit66may also use additional techniques in some examples in combination with any of temporal level, decoding order, or display order to determine the previous picture.

As shown inFIG. 2, video encoder20is one example of a video encoder configured to encode an open decoding refresh (ODR) picture; determine a previously coded picture having a display order value less than a display order value of the ODR picture and having a temporal identifier value equal to zero; and signal syntax data for the ODR picture representative of an identifier of the determined previously coded picture to cause a video decoder to determine whether the ODR picture is being used for random access based on the identifier of the determined previously coded picture.

FIG. 3is a block diagram illustrating an example of video decoder30that may implement techniques for improved management of a decoding picture buffer (DPB) when pictures are used for random access. In the example ofFIG. 3, video decoder30includes an entropy decoding unit70, motion compensation unit72, intra prediction unit74, inverse quantization unit76, inverse transformation unit78, reference picture memory82and summer80. In some examples, reference picture memory82may be referred to in this disclosure as a picture buffer or decoding picture buffer. Video decoder30may, in some examples, perform a decoding pass generally reciprocal to the encoding pass described with respect to video encoder20(FIG. 2). Motion compensation unit72may generate prediction data based on motion vectors received from entropy decoding unit70, while intra-prediction unit74may generate prediction data based on intra-prediction mode indicators received from entropy decoding unit70.

During the decoding process, video decoder30receives an encoded video bitstream that represents video blocks of an encoded video slice and associated syntax elements from video encoder20. Entropy decoding unit70of video decoder30entropy decodes the bitstream to generate quantized coefficients, motion vectors or intra-prediction mode indicators, and other syntax elements. Entropy decoding unit70forwards the motion vectors to and other syntax elements to motion compensation unit72. Video decoder30may receive the syntax elements at the video slice level and/or the video block level.

When the video slice is coded as an intra-coded (I) slice, intra prediction unit74may generate prediction data for a video block of the current video slice based on a signaled intra prediction mode and data from previously decoded blocks of the current frame or picture. When the video picture is coded as an inter-coded (i.e., B, P or GPB) slice, motion compensation unit72produces predictive blocks for a video block of the current video slice based on the motion vectors and other syntax elements received from entropy decoding unit70. The predictive blocks may be produced from one of the reference pictures within one of the reference picture lists. Video decoder30may construct the reference picture lists, List0and List1, using default construction techniques based on reference pictures stored in reference picture memory92. Motion compensation unit72determines prediction information for a video block of the current video slice by parsing the motion vectors and other syntax elements, and uses the prediction information to produce the predictive blocks for the current video block being decoded. For example, motion compensation unit72uses some of the received syntax elements to determine a prediction mode (e.g., intra- or inter-prediction) used to code the video blocks of the video slice, an inter-prediction slice type (e.g., B slice, P slice, or GPB slice), construction information for one or more of the reference picture lists for the slice, motion vectors for each inter-encoded video block of the slice, inter-prediction status for each inter-coded video block of the slice, and other information to decode the video blocks in the current video slice.

Motion compensation unit72may also perform interpolation based on interpolation filters. Motion compensation unit72may use interpolation filters as used by video encoder20during encoding of the video blocks to calculate interpolated values for sub-integer pixels of reference blocks. In this case, motion compensation unit72may determine the interpolation filters used by video encoder20from the received syntax elements and use the interpolation filters to produce predictive blocks.

Inverse quantization unit76inverse quantizes, i.e., de-quantizes, the quantized transform coefficients provided in the bitstream and decoded by entropy decoding unit80. The inverse quantization process may include use of a quantization parameter QPYcalculated by video decoder30for each video block in the video slice to determine a degree of quantization and, likewise, a degree of inverse quantization that should be applied.

Inverse transform unit78applies an inverse transform, e.g., an inverse DCT, an inverse integer transform, or a conceptually similar inverse transform process, to the transform coefficients in order to produce residual blocks in the pixel domain.

After motion compensation unit82generates the predictive block for the current video block based on the motion vectors and other syntax elements, video decoder30forms a decoded video block by summing the residual blocks from inverse transform unit78with the corresponding predictive blocks generated by motion compensation unit82. Summer90represents the component or components that perform this summation operation. If desired, a deblocking filter may also be applied to filter the decoded blocks in order to remove blockiness artifacts. Other loop filters (either in the coding loop or after the coding loop) may also be used to smooth pixel transitions, or otherwise improve the video quality. The decoded video blocks in a given frame or picture are then stored in reference picture memory92, which stores reference pictures used for subsequent motion compensation. Reference picture memory82also stores decoded video for later presentation on a display device, such as display device32ofFIG. 1.

As described above, video data of pictures decoded by video encoder30may be stored in reference picture memory82. Decoding control unit84may perform memory management control operations to store and remove video data of reference pictures stored in reference picture memory82. In some examples, decoding control unit84may implement techniques of the present disclosure to improve management of a reference picture memory in a video decoder when pictures are used for random access.

To illustrate an example of random access, a user may request to view a video that is displayed by a display device. An application, such as a web browser or a media player, may enable the user to control the display of the video. The video may be displayed based on pictures that are decoded by video decoder30. In the current example, a user may provide input to randomly access a different location of the video, such that the input is received by the application. The application may identify a picture associated with the selected location and request the picture, e.g., from a source device during network streaming or from a storage medium accessible by an input interface. The application may request encoded video data that begins with the RAP picture. Video decoder30may receive the RAP picture from the application.

Video decoder30may use ODR or IDR pictures to provide random access to various locations of a coded video bitstream. Examples of an independently decodable pictures that video decoder30may use to provide random access include instantaneous decoding refresh (IDR) and open decoding refresh (ODR) RAP pictures. As previously described, an IDR picture is an independently decodable I-picture of a closed GOP, while an ODR picture is an I-picture of an open GOP.

In accordance with techniques of the present disclosure, decoding control unit84may initially receive the ODR RAP picture in a stream of encoded video for decoding. Decoding control unit84may determine whether the ODR RAP picture is an ODR picture. For instance, decoding control unit84may determine whether a syntax element in the slice header of a slice in the picture indicates that the picture is an ODR picture. As one example, an OdrPicFlag variable stored in the slice header of a slice in the ODR picture may be set to a value of true in the slice header to indicate the picture is an ODR picture. When decoding control unit84determines that the ODR RAP picture is an ODR picture, decoding control unit84may decode the picture.

Upon decoding the ODR RAP picture, decoding control unit84may determine an identifier of the previous picture that is stored in the header of a slice in the ODR picture. For instance, decoding control unit84may identify a syntax element in the slice header of a slice in the ODR picture that identifies the previous picture. As previously described inFIG. 1, the ODR picture may include a pre_pic_POC syntax element, which indicates a display order value of the previous picture. Decoding control unit84may then determine whether the previous picture is stored in a reference picture memory of decoding control unit84by comparing the value of the pre_pic_POC syntax element with display order values of pictures stored in the reference picture memory to identify a match.

In the current example, decoding control unit82may determine that reference picture memory82does not include the previous picture identified by the identifier stored in the ODR picture. Consequently, decoding control unit84may determine that the ODR picture is being used as an ODR RAP. Decoding control unit84may therefore set a random access status variable to true because decoding control unit84has determined that the ODR picture is being used for random access.

Decoding control unit84, in response to determining an ODR picture is being used for random access, may mark pictures in reference picture memory82as unused for reference after decoding the ODR picture. In some examples, decoding control unit84may mark all pictures as unused. After decoding the ODR picture, decoding control unit84may leave the random access status unchanged. Decoding control unit84may subsequently delete the pictures from reference picture memory82that have been marked with the unused status.

In this manner, video decoder30ofFIG. 3represents an example of a video decoder configured to determine, from data associated with an open decoding refresh (ODR) picture of video data, an identifier for a previous picture of the video data; determine whether the previous picture is currently stored in a reference picture memory; and, decode only data for pictures of the video data having display order values that are greater than a display order value of the ODR picture when the previous picture is not stored in the reference picture memory.

When the random access status indicates decoding control unit84is performing random access, decoding control unit84may decode video data of pictures with display orders that are greater than a display order value of the ODR picture. For instance, when video decoder30uses an ODR picture as an ODR RAP (e.g., for random access), decoding control unit84may further create a variable to store the POC value of the ODR RAP picture. In one example, the variable may be named “CurrODRPOC.” Decoding control unit84, upon determining that an ODR picture is being used an ODR RAP, may set CurrODRPOC to the POC value of the ODR picture.

Decoding control unit84may parse the slice headers of slices included in each NAL unit that is subsequently received from source device12. For each NAL unit, decoding control unit84may determine whether the POC value of the slice included in the NAL unit. For instance, in one example, decoding control unit84may compare the POC value of a slice included in a NAL unit to the POC value stored in CurrODRPOC. If the POC value of the slice is smaller than the POC value of the ODR RAP picture, decoding control unit84may skip decoding the encoded video data of the slice. In this way, such NAL units are not further decoded. Consequently, video decoder30may refrain from decoding video data of NAL units that are not required to decode from the selected ODR RAP picture. In some examples, in response to determining that the ODR picture is being used as a random access point, video decoder30may skip outputting of data for pictures having display order values less than a display order value of the ODR picture and decoding order values greater than a decoding order value of the ODR picture Thus, techniques of the present disclosure may reduce unnecessary decoding and outputting of video data when decoding control unit84determines that an ODR RAP picture has been selected for random access.

In one example, decoding control unit84may determine that the POC value of the slice is larger than the POC value of the ODR RAP picture. In such examples, the encoded video of the slice subsequently decoded. In response to determining that the POC value of the slice in a NAL unit is larger than the POC value of the ODR RAP picture, decoding control unit84may also change the random access status to indicate decoding of pictures for normal, sequential display of pictures rather than random access. For example, decoding control unit84may set the random access status variable to indicate decoding of pictures for normal, sequential display of pictures.

FIG. 4is a conceptual diagram that illustrates one example of video fragment100that includes a closed group of pictures (GOP)102. A GOP may be described as a grouping of sequential (in presentation order) pictures of a video sequence. A video fragment may include any number of GOPs similar to GOP102.

The example ofFIG. 4depicts a portion of a video fragment100. The video fragment includes a plurality of pictures104A-104K. Fragment100may comprise one or more groups of pictures (GOPs), such as GOP102. The one or more GOPs102may be described as a grouping of pictures of a video representation.

GOP102illustrated inFIG. 4is an example of a closed GOP. That is, each of pictures104B-104I can be decoded without reference to pictures external to GOP102, e.g., pictures104A,104J, and104K. As shown inFIG. 2, picture104A precedes (in presentation order) the pictures104B-104I of GOP102. Pictures104J-104K are subsequent to (in presentation order) pictures104B-104I of GOP102. Pictures104A and104J-104K may be part of another GOP of fragment100.

A presentation order of pictures104A-104K may be different than a decoding order of the pictures. For example, a GOP may include any combination of I, B, or P-pictures. Intra-coded pictures (I-pictures) are those pictures that are independently decodable, meaning that to decode an I-picture, a decoder need not rely on content of other pictures. In the example ofFIG. 4, pictures104A and104I are examples of I-pictures. P-pictures may be inter-coded relative to one or more pictures in one direction. In the example ofFIG. 4, pictures104B,104C, and104E are examples of P-pictures. B-pictures may be inter-coded relative to one or more pictures in two directions. In the example ofFIG. 4, pictures104D,104F,104G, and104H are examples of B-pictures.

As discussed above, according to the example ofFIG. 4, a decoding order of pictures104A-104K may be different than a presentation order of the pictures. For example, when decoding pictures104B-104I, picture104I (an I-picture) may be decoded first. As indicated by the arrow108, picture104E relies on content of picture104I to be correctly decoded. As such, picture104E may be decoded after picture104I is decoded.

As indicated by arrow106, picture104C may rely on content of picture104E to be correctly decoded. As indicated by arrow110, picture104G may rely on content of both pictures104E and picture104I to be correctly decoded. As such, in some examples, decoding of pictures104C and104G may occur after decoding of pictures104I and104E. Pictures104B,104D,104F, and104H each rely on content of one or more of pictures104C,104E,104G, and104I, respectively, and therefore may be decoded after pictures104C,104E,104G, and104I have been decoded.

As described above, an instantaneous decoding refresh (IDR) access point may be described as an access point of a closed GOP, e.g., GOP102inFIG. 4. A GOP including only pictures that are correctly decodable without relying on content of pictures outside of the GOP may be considered a closed GOP102.

FIG. 4depicts two examples of IDR access points. As shown in theFIG. 2example, picture104A does not rely on the content of any other picture to be correctly decodable, i.e., picture104A does not include any arrow indicating reliance on another picture. Picture104A may be considered a GOP in and of itself, because there are no pictures preceding picture104A. As such, picture104A may be considered an IDR access point, because picture104A is an access point of a GOP that does not rely on the content of any pictures outside the GOP (consisting only of picture104A) to be correctly decoded.

Picture104I may also be considered an IDR access point of closed GOP102. As shown in theFIG. 4, for example, picture104I is an I-picture that is independently decodable without relying on the content of any other picture (e.g., pictures104B-104H) of GOP102. Although each of pictures104B-104H rely on the content of other pictures within GOP102to be correctly decoded as described above, none of pictures104B-104H rely on the content of any pictures outside of GOP102. As such, GOP102may be considered a closed GOP that includes an IDR access point, namely picture104I.

In accordance with techniques of the present disclosure, video decoder30as shown inFIGS. 1 and 3may initially decode pictures for normal, sequential display. Video decoder30may therefore set a random access status to indicate that pictures are decoded for normal, sequential display. At a later time, video decoder30may receive an indication to randomly access picture104F of video fragment100. Consequently, video decoder30may identify the closest independently decodable picture usable to properly decode and display picture104F.

In the current example, picture104F depends on IDR picture104I to be properly decoded, which is also the closest independently decodable picture to picture104F in decoding order. Video decoder30may therefore determine that IDR picture104I may be used for random access. Consequently, video decoder30may send a request to video encoder20for IDR picture104I and subsequently decodable pictures that follow IDR picture104I in decoding order. In response to receiving IDR picture104I, video decoder30may determine whether IDR picture104I is an IDR or ODR picture. Because picture104I is an IDR picture, video decoder30leaves the random access status unchanged. Since the random access status indicates that pictures are decoded for normal, sequential display, video decoder30decodes IDR picture104I and pictures104E,104G and104F. Video decoder30may then send decoded picture104F to a display device for display.

FIG. 5is a conceptual diagram that illustrates one example of at least a portion of a video fragment150that includes an open GOP152that includes an open decoding refresh (ODR) access point. Similar to the example ofFIG. 4, picture154A is an I-picture and an IDR access point. Also similar to the example ofFIG. 4, picture1541is an I-picture corresponding to a random access point. However, theFIG. 5example differs from theFIG. 4example, in that pictures of GOP152prior to I-picture1541in display order rely on the content of picture154A in order to be correctly decodable. For example, as indicated by directional arrows, each of pictures154B,154C, and154E directly rely on content of picture154A. Pictures154D, and154F-154H each rely indirectly on the content of picture154A, as each rely at least in part on the content of one or more of pictures154B,154C, and154E to be correctly decoded. However, as also depicted inFIG. 5, pictures154J and154K, which follow I-picture1541in display order, may be correctly decoded without reliance on any pictures prior to I-picture1541. Thus, I-picture1541may be an ODR used for random access.

In accordance with techniques of the present disclosure, video decoder30as shown inFIGS. 1 and 3may initially decode pictures for normal, sequential display. Video decoder30may therefore set a random access status to indicate that pictures are decoded for normal, sequential display. At a later time, video decoder30may receive an indication to randomly access picture154C of video fragment150. For instance, a client device that includes video decoder30may receive an indication to begin display of a video at picture154F. Consequently, video decoder30may identify the closest independently decodable picture usable to properly decode and display picture154F.

In the current example, picture154F depends on ODR picture1541to be properly decoded, which is also the closest independently decodable picture to picture154F in decoding order. Video decoder30may therefore determine that ODR picture1541may be used for random access. Consequently, video decoder30may send a request to video encoder20for ODR picture1541and subsequently decodable pictures that follow ODR picture1541in decoding order. In response to receiving ODR picture1541, video decoder30may determine whether ODR picture1541is an IDR or ODR picture.

In the current example, video decoder30, in response to determining an ODR picture is being used as an ODR RAP for random access, may set a random access status variable to true and mark pictures in the reference picture memory as unused for reference after decoding the ODR picture. Video decoder30may subsequently delete the pictures from the reference picture memory having the unused status.

As subsequent pictures are received while the random access variable is set to true, video decoder30may decode only data for pictures of the video data having display order values that are greater than a display order value of the ODR picture. For instance, when ODR picture1541is being used for random access, video decoder30may further create a variable to store the POC value of the ODR RAP picture, such as CurrODRPOC as described inFIG. 1. Video decoder30, upon determining that ODR picture1541is being used an ODR RAP, may set CurrODRPOC to the POC value of the ODR picture. As video decoder30receives subsequent NAL units from source device12, video decoder30may parse the slice headers of slices included in each NAL unit.

For each NAL unit, video decoder30may determine whether the POC value of the slice included in the NAL unit. For instance, in one example, video decoder30may compare the POC value of a slice included in a NAL unit to the POC value stored in CurrODRPOC. If the POC value of the slice is smaller than the POC value of the ODR RAP picture, video decoder30may skip decoding the encoded video data in the NAL unit. In this way, video decoder30may refrain from decoding video data of NAL units that are not required to decode from the selected ODR RAP picture. Video decoder30may also, in response to determining that an ODR picture is being used for random access, may skip outputting of data for pictures having display order values less than a display order value of the ODR picture and decoding order values greater than a decoding order value of the ODR picture. In the current example, video decoder30may therefore decode ODR picture1541and pictures154E,154G and154F. Video decoder30may then send decoded picture154F to a display device for display.

FIG. 6is a conceptual diagram of an ODR picture and a previously encoded picture identified in the ODR picture, in accordance with techniques of the disclosure.FIG. 6includes open GOP170and a closed GOP172. As shown inFIG. 6, closed GOP170includes picture174, which may be I-picture. Closed GOP172includes pictures176. Included in pictures176are B-pictures176A-176C and ODR picture176D, which may be an independent decodable picture (e.g., I-picture). Initially, video encoder20(e.g., as shown inFIGS. 1 and 2) may encode ODR picture176D in a video sequence. When video encoder20encodes ODR picture176D, video encoder20may determine previously encoded picture174that has a display order that is less than the display order of ODR picture176D. In some examples, video encoder20may search a reference picture memory to identify previously encoded picture174. Video encoder20may determine previous picture174by determining whether one or more pictures of open GOP172depend on previous picture174. For instance, pictures176A-176C (e.g., leading pictures) of open GOP172that includes encoded ODR picture176D may depend on previous picture174for proper decoding. In some examples, video encoder20may determine as the previously encoded picture, picture174that is the closest picture prior to ODR picture in decoding and display order that has a temporal level equal to 0. Unlike previous picture174, pictures176A-176C may have temporal levels that are greater than zero and therefore video encoder20may not select any of these pictures as the previous picture.

When video encoder20determines previous picture174, video encoder20may signal an identifier of previous picture174in a header of a slice in ODR picture176D. A header may be a slice header or picture parameter set. In some examples, the identifier may be a Picture Order Count (POC) value that indicates the display order of previous picture174. Video encoder20may store the POC value of previous picture174in the slice header of a slice in ODR picture176D. In one example, the slice header may include a syntax element identified by the name “pre_pic_POC.” The pre_pic_POC value may specify the POC value of the previous picture determined by video encoder20.

After video encoder20has stored the POC value of previous picture174in the header a slice in of ODR picture176D, an encapsulation unit may further encapsulate ODR picture176D in a NAL unit. An output interface may receive the NAL unit from the encapsulation unit and may subsequently send the NAL unit to a destination device along with other NAL units that include encoded video data for decoding.

FIG. 7is a flowchart illustrating operations that may be implemented by a video decoder (e.g., as shown inFIGS. 1 and 3), in accordance with techniques of the present disclosure. The example operations ofFIG. 7will be described with respect to video decoder30as shown inFIGS. 1 and 3. As shown inFIG. 7, video decoder30may initially begin playing a video sequence (190). For example, a user may provide input to begin playback of video data at the start of a video sequence.

At a later point in time, the user may navigate to an out-of-order temporal location of the video sequence (192). In one example, the out-of-order temporal location may be a different picture of the video sequence that is not the next picture in display order. In such examples, video decoder30may therefore randomly access another picture in the video sequence. Video decoder30may apply techniques of the present disclosure to determine whether random access has occurred (194).

Video decoder30may perform reference picture memory management techniques based on random access as described in this disclosure (196). Operations performed to determine whether random access has occurred and perform reference picture memory management techniques are further described inFIGS. 8A and 8B. In the techniques ofFIGS. 8A and 8B, video decoder30may mark pictures in a reference picture memory as unused and further decode only pictures necessary to begin playback at the randomly access picture of the video sequence. Upon determining whether random access has occurred and subsequently applying techniques of the present disclosure, video decoder30may begin playing the different picture of the video sequence (198). In this way, video decoder30may perform random access to access a different picture of the video sequence. Techniques to perform random access are now further described inFIGS. 8A and 8B.

FIGS. 8A and 8Bare flowcharts that illustrate examples of operations that may be performed by a video decoder in accordance with techniques of the present disclosure. The example operations ofFIG. 7will be described with respect to video decoder30as shown inFIGS. 1 and 3. As shown inFIG. 8A, video decoder30may initially receive an indication (e.g., a user input) to decode a picture that does not follow the most recently decoded picture in decoding order. Consequently, video decoder30may determine the encoded picture identified by the indication (210). Video decoder30may determine whether the encoded picture is an ODR picture (212). If video decoder30determines the encoded picture is an ODR picture, video decoder30may decode the ODR picture (214).

Upon decoding the ODR picture, video decoder30may determine whether the reference picture memory (e.g., a decoding picture buffer) of video decoder30includes a previously encoded picture that is identified by the ODR picture (216). If the reference picture memory includes the previously encoded picture, video decoder30applies one or more conventional reference picture memory techniques to the reference picture memory and continues to selecting a subsequent encoded picture for decoding (210).

If the reference picture memory does not include the previously encoded picture identified by the ODR picture, video decoder30may set a random access status variable to indicate video decoder30is performing random access (218). Video decoder30may then mark pictures included in the reference picture memory as unused for reference (220). In this way, video decoder30may later remove the pictures marked as unused from the reference picture memory. After marking the pictures as unused for reference, video decoder30may store the decoded ODR picture in the reference picture memory (222). Video decoder30then determines the next encoded picture for decoding (210).

FIG. 8Billustrates operations that may be performed by a video decoder when an encoded picture to be decoded by the video decoder is not an ODR picture. When an encoded picture determined for decoding by video decoder30is not an ODR picture, video decoder30determines whether the random access status variable indicates that video decoder30is performing random access (230). If the random access status variable indicates video decoder30is not performing random access, video decoder30decodes the picture (232). Video decoder30may, output data for picture having a display order value that is greater than a display order value of the ODR picture. Video decoder30may then determine the next encoded picture in the video sequence for decoding (238).

As shown inFIG. 8B, when video decoder30determines the random access status variable indicates that video decoder30is performing random access, video decoder30may determine whether the display order of the picture determined for decoding is less than the display order of the ODR picture (232). In this manner, video decoder30may determine that an open decoding refresh (ODR) picture of video data is being used as a random access point. If the display order of the picture selected for decoding is less than the display order of the ODR picture, video decoder30may leave the decoding status variable unchanged to indicate that video is performing random access. Consequently, video decoder30refrains from further decoding the selected picture. Video decoder30may therefore refrain from outputting data for pictures having display order values less than a display order value of the ODR picture and decoding order values greater than a decoding order value of the ODR picture. Video decoder30may then determine the next encoded picture for decoding (238).

In some examples, video decoder30may determine that the display order of the picture selected for decoding is greater than the display order of the ODR picture (232). In this manner, video decoder30may determine that an open decoding refresh (ODR) picture of video data is being used as a random access point. Consequently, video decoder30may set the random access status variable to indicate that video decoder30is no longer performing random access, i.e., video decoder30is performing normal decoding of video pictures (234). After setting the random access status variable to indicate video decoder30is performing normal decoding of video pictures, video decoder30may decode the selected picture (236). Video decoder30may then determine a next encoded picture for decoding (238).

In this manner, the method ofFIGS. 8A and 8Brepresents an example of a method that includes determining that an open decoding refresh (ODR) picture of video data is being used as a random access point; and based on the determination, skipping output of data for pictures having display order values less than a display order value of the ODR picture and decoding order values greater than a decoding order value of the ODR picture.

FIG. 9illustrates operations that may be performed by a video encoder to signal a previous picture in syntax data of ODR picture to enable random access techniques of the present disclosure. As shown inFIG. 9, a video encoder, such as video encoder20inFIGS. 1 and 2, may encode an ODR picture (250). Video encoder20may determine a picture that was previous encoded by video encoder20(252). Upon determining the picture, video encoder20may determine whether the previously coded picture has a display order that is less than the display order of the ODR picture (254). Video encoder20may further determine whether the temporal level value of the previously encoded picture is zero. When the previously coded picture has a display order that is less than the display order of ODR picture and a temporal level of zero, video encoder20may signal syntax data for the ODR picture that identifies the previously coded picture (256). By signaling the previously coded picture in syntax data of the ODR picture, a video decoder may subsequently use the syntax data when performing random access techniques as described in the disclosure. In some examples, video encoder20may determine the display order of the determined picture is not less than the display order of the ODR picture. In such examples, video encoder20may determine another previously coded picture, and subsequently determine whether the display order is less than the ODR picture and has a temporal level equal to zero.