Patent Publication Number: US-11665219-B2

Title: Processing media data using a generic descriptor for file format boxes

Description:
This application claims the benefit of U.S. Provisional Application No. 62/530,761, filed Jul. 10, 2017, the entire contents of which are incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to storage and transport of encoded video data. 
     BACKGROUND 
     Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless broadcast systems, personal digital assistants (PDAs), laptop or desktop computers, digital cameras, digital recording devices, digital media players, video gaming devices, video game consoles, cellular or satellite radio telephones, video teleconferencing devices, and the like. Digital video devices implement video compression techniques, such as those described in the standards defined by MPEG-2, MPEG-4, ITU-T H.263 or ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), ITU-T H.265 (also referred to High Efficiency Video Coding (HEVC)), and extensions of such standards, to transmit and receive digital video information more efficiently. 
     After video data has been encoded, the video data may be packetized for transmission or storage. The video data may be assembled into a video file conforming to any of a variety of standards, such as the International Organization for Standardization (ISO) base media file format and extensions thereof, such as AVC. 
     SUMMARY 
     In general, this disclosure describes various example designs for a generic descriptor for file format boxes, which can be used to conveniently expose file format level information into a media presentation description (MPD) of dynamic adaptive streaming over HTTP (DASH), session description protocol (SDP), and other such streaming protocols. The designs provide an automated descriptor generation from file format boxes to avoid complex definition later and also avoid mismatches. Although the ideas are described in the context of DASH and ISO base media file format, the mechanism applies to other media container file formats and other media delivery format and protocols. 
     In one example, a method of retrieving media data includes processing a descriptor of a manifest file of media content, the descriptor corresponding to an adaptation set of the media content and including data representative of a box of file level information that describes a property of a track of a media file of the media content including media data of the adaptation set, determining whether to retrieve the media data of the adaptation set according to the data representative of the file level information, and in response to determining to retrieve the media data, sending a request to retrieve the media data. 
     In another example, a device for retrieving media data includes a memory configured to store media data, and a processor implemented in circuitry and configured to process a descriptor of a manifest file of media content including the media data, the descriptor corresponding to an adaptation set of the media content and including data representative of a box of file level information that describes a property of a track of a media file of the media content including media data of the adaptation set, determine whether to retrieve the media data of the adaptation set according to the data representative of the file level information, and in response to determining to retrieve the media data, send a request to retrieve the media data. 
     In another example, a device for retrieving media data includes means for processing a descriptor of a manifest file of media content, the descriptor corresponding to an adaptation set of the media content and including data representative of a box of file level information that describes a property of a track of a media file of the media content including media data of the adaptation set, means for determining whether to retrieve the media data of the adaptation set according to the data representative of the file level information, and means for sending a request to retrieve the media data in response to determining to retrieve the media data. 
     In another example, a computer-readable storage medium has stored thereon instructions that, when executed, cause a processor to process a descriptor of a manifest file of media content, the descriptor corresponding to an adaptation set of the media content and including data representative of a box of file level information that describes a property of a track of a media file of the media content including media data of the adaptation set, determine whether to retrieve the media data of the adaptation set according to the data representative of the file level information, and send a request to retrieve the media data in response to determining to retrieve the media data. 
     In another example, a method of generating media data includes processing a box of file level information that describes a property of a track of a media file of media content including media data, generating a descriptor for a manifest file of the media content, the descriptor corresponding to an adaptation set of the media content and including data representative of the box of the file level information, the media data of the track being included in the adaptation set, and sending the manifest file including the descriptor to a client device. 
     In another example, a device for generating media data includes a memory configured to store media data, and a processor implemented in circuitry and configured to process a box of file level information that describes a property of a track of a media file of media content including the media data, generate a descriptor for a manifest file of the media content, the descriptor corresponding to an adaptation set of the media content and including data representative of the box of the file level information, the media data of the track being included in the adaptation set, and send the manifest file including the descriptor to a client device. 
     In another example, a device for generating media data includes means for processing a box of file level information that describes a property of a track of a media file of media content including media data, means for generating a descriptor for a manifest file of the media content, the descriptor corresponding to an adaptation set of the media content and including data representative of the box of the file level information, the media data of the track being included in the adaptation set, and means for sending the manifest file including the descriptor to a client device. 
     In another example, a computer-readable storage medium has stored thereon instructions that, when executed, cause a processor to process a box of file level information that describes a property of a track of a media file of media content including media data, generate a descriptor for a manifest file of the media content, the descriptor corresponding to an adaptation set of the media content and including data representative of the box of the file level information, the media data of the track being included in the adaptation set, and send the manifest file including the descriptor to a client device. 
     The details of one or more examples are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a block diagram illustrating an example system that implements techniques for streaming media data over a network. 
         FIG.  2    is a block diagram illustrating an example set of components of a retrieval unit. 
         FIG.  3    is a conceptual diagram illustrating elements of example multimedia content. 
         FIG.  4    is a block diagram illustrating elements of an example video file, which may correspond to a segment of a representation. 
         FIG.  5    is a conceptual diagram illustrating an example content model for DASH multitrack. 
         FIG.  6    is a conceptual diagram illustrating an example client model. 
         FIG.  7    is a flowchart illustrating an example method for generating media data and for retrieving media data in accordance with the techniques of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The techniques of this disclosure may be applied to video files conforming to video data encapsulated according to any of ISO base media file format (ISOBMFF), extensions to ISOBMFF, Scalable Video Coding (SVC) file format, Advanced Video Coding (AVC) file format, High Efficiency Video Coding (HEVC) file format, Third Generation Partnership Project (3GPP) file format, and/or Multiview Video Coding (MVC) file format, or other video file formats. A draft of ISO BMFF is specified in ISO/IEC 14496-12, available from phenix.int-evry.fr/mpeg/doc_end_user/documents/111_Geneva/wg11/w15177-v6-w15177.zip. A draft of another example file format, MPEG-4 file format, is specified in ISO/IEC 14496-15, available from wg11.sc29.org/doc_end_user/documents/115_Geneva/wg11/w16169-v2-w16169.zip. 
     ISOBMFF is used as the basis for many codec encapsulation formats, such as the AVC file format, as well as for many multimedia container formats, such as the MPEG-4 file format, the 3GPP file format (3GP), and the digital video broadcasting (DVB) file format. 
     In addition to continuous media, such as audio and video, static media, such as images, as well as metadata can be stored in a file conforming to ISOBMFF. Files structured according to the ISOBMFF may be used for many purposes, including local media file playback, progressive downloading of a remote file, segments for Dynamic Adaptive Streaming over HTTP (DASH), containers for content to be streamed and its packetization instructions, and recording of received real-time media streams. 
     A box is an elementary syntax structure in ISOBMFF, including a four-character coded box type, the byte count of the box, and the payload. An ISOBMFF file includes a sequence of boxes, and boxes may contain other boxes. According to ISOBMFF, a Movie box (“moov”) contains the metadata for the continuous media streams present in the file, each one represented in the file as a track. Per ISOBMFF, metadata for a track is enclosed in a Track box (“trak”), while the media content of a track is either enclosed in a Media Data box (“mdat”) or provided directly in a separate file. The media content for tracks includes a sequence of samples, such as audio or video access units. 
     ISOBMFF specifies the following types of tracks: a media track, which contains an elementary media stream, a hint track, which either includes media transmission instructions or represents a received packet stream, and a timed metadata track, which comprises time-synchronized metadata. 
     Although originally designed for storage, the ISOBMFF has proven to be very valuable for streaming, e.g., for progressive download or DASH. For streaming purposes, movie fragments defined in ISOBMFF can be used. 
     The metadata for each track includes a list of sample description entries, each providing the coding or encapsulation format used in the track and the initialization data needed for processing that format. Each sample is associated with one of the sample description entries of the track. 
     The ISOBMFF enables specifying sample-specific metadata with various mechanisms. Specific boxes within the Sample Table box (“stbl”) have been standardized to respond to common needs. For example, a Sync Sample box (“stss”) is used to list the random access samples of the track. The sample grouping mechanism enables mapping of samples according to a four-character grouping type into groups of samples sharing the same property specified as a sample group description entry in the file. Several grouping types have been specified in the ISOBMFF. 
     Virtual reality (VR) is the ability to be virtually present in a virtual, non-physical world created by the rendering of natural and/or synthetic images and sounds correlated by movements of an immersed user, allowing interaction with that virtual world. With recent progress made in rendering devices, such as head mounted displays (HMD) and VR video (often also referred to as 360-degree video) creation, a significant quality of experience can be offered. VR applications include gaming, training, education, sports video, online shopping, entrainment, and so on. 
     A typical VR system includes the following components and steps:
         1) A camera set, which typically includes multiple individual cameras pointing in different directions, ideally collectively covering all viewpoints around the camera set.   2) Image stitching, where video pictures taken by the multiple individual cameras are synchronized in the time domain and stitched in the space domain, to be a spherical video, but mapped to a rectangular format, such as equi-rectangular (like a world map) or cube map.   3) The video in the mapped rectangular format is encoded/compressed using a video codec, e.g., H.265/HEVC or H.264/AVC.   4) The compressed video bitstream(s) may be stored and/or encapsulated in a media format and transmitted (possibly only the subset covering the area being seen by a user, sometimes referred to as the viewport) through a network to a receiving device (e.g., a client device).   5) The receiving device receives the video bitstream(s) or part thereof, possibly encapsulated in a file format, and sends the decoded video signal or part thereof to a rendering device (which may be included in the same client device as the receiving device).   6) The rendering device can be, e.g., an HMD, which can track head movement and even eye move moment, and may render the corresponding part of the video such that an immersive experience is delivered to the user.       

     Omnidirectional MediA Format (OMAF) is being developed by the Moving Pictures Experts Group (MPEG) to define a media format that enables omnidirectional media applications, focusing on VR applications with 360-degree video and associated audio. OMAF specifies a list of projection methods that can be used for conversion of a spherical or 360-degree video into a two-dimensional rectangular video, followed by how to store omnidirectional media and the associated metadata using the ISO base media file format (ISOBMFF) and how to encapsulate, signal, and stream omnidirectional media using dynamic adaptive streaming over HTTP (DASH), and finally, which video and audio codecs, as well as media coding configurations, can be used for compression and playback of the omnidirectional media signal. OMAF is to become ISO/IEC 23090-2, and a draft specification is available from wg11.sc29.org/doc_end_user/documents/119_Torino/wg11/m40849-v1-m40849_OMAF_text_Berlin_output.zip. 
     In HTTP streaming protocols, such as DASH, frequently used operations include HEAD, GET, and partial GET. The HEAD operation retrieves a header of a file associated with a given uniform resource locator (URL) or uniform resource name (URN), without retrieving a payload associated with the URL or URN. The GET operation retrieves a whole file associated with a given URL or URN. The partial GET operation receives a byte range as an input parameter and retrieves a continuous number of bytes of a file, where the number of bytes correspond to the received byte range. Thus, movie fragments may be provided for HTTP streaming, because a partial GET operation can get one or more individual movie fragments. In a movie fragment, there can be several track fragments of different tracks. In HTTP streaming, a media presentation may be a structured collection of data that is accessible to the client. The client may request and download media data information to present a streaming service to a user. 
     DASH is specified in ISO/IEC 23009-1, and is a standard for HTTP (adaptive) streaming applications. ISO/IEC 23009-1 mainly specifies the format of the media presentation description (MPD), also known as a manifest or manifest file, and media segment formats. The MPD describes the media available on a server and allows a DASH client to autonomously download an appropriate media version at an appropriate media time. 
     In the example of streaming 3GPP data using HTTP streaming, there may be multiple representations for video and/or audio data of multimedia content. As explained below, different representations may correspond to different coding characteristics (e.g., different profiles or levels of a video coding standard), different coding standards or extensions of coding standards (such as multiview and/or scalable extensions), or different bitrates. The manifest of such representations may be defined in a Media Presentation Description (MPD) data structure. A media presentation may correspond to a structured collection of data that is accessible to an HTTP streaming client device. The HTTP streaming client device may request and download media data information to present a streaming service to a user of the client device. A media presentation may be described in the MPD data structure, which may include updates of the MPD. 
     A media presentation may contain a sequence of one or more Periods. Each period may extend until the start of the next Period, or until the end of the media presentation, in the case of the last period. Each period may contain one or more representations for the same media content. A representation may be one of a number of alternative encoded versions of audio, video, timed text, or other such data. The representations may differ by encoding types, e.g., by bitrate, resolution, and/or codec for video data and bitrate, language, and/or codec for audio data. The term representation may be used to refer to a section of encoded audio or video data corresponding to a particular period of the multimedia content and encoded in a particular way. 
     Representations of a particular period may be assigned to a group indicated by an attribute in the MPD indicative of an adaptation set to which the representations belong. Representations in the same adaptation set are generally considered alternatives to each other, in that a client device can dynamically and seamlessly switch between these representations, e.g., to perform bandwidth adaptation. For example, each representation of video data for a particular period may be assigned to the same adaptation set, such that any of the representations may be selected for decoding to present media data, such as video data or audio data, of the multimedia content for the corresponding period. The media content within one period may be represented by either one representation from group 0, if present, or the combination of at most one representation from each non-zero group, in some examples. Timing data for each representation of a period may be expressed relative to the start time of the period. 
     A representation may include one or more segments. Each representation may include an initialization segment, or each segment of a representation may be self-initializing. When present, the initialization segment may contain initialization information for accessing the representation. In general, the initialization segment does not contain media data. A segment may be uniquely referenced by an identifier, such as a uniform resource locator (URL), uniform resource name (URN), or uniform resource identifier (URI). The MPD may provide the identifiers for each segment. In some examples, the MPD may also provide byte ranges in the form of a range attribute, which may correspond to the data for a segment within a file accessible by the URL, URN, or URI. 
     Different representations may be selected for substantially simultaneous retrieval for different types of media data. For example, a client device may select an audio representation, a video representation, and a timed text representation from which to retrieve segments. In some examples, the client device may select particular adaptation sets for performing bandwidth adaptation. That is, the client device may select an adaptation set including video representations, an adaptation set including audio representations, and/or an adaptation set including timed text. Alternatively, the client device may select adaptation sets for certain types of media (e.g., video), and directly select representations for other types of media (e.g., audio and/or timed text). 
     A typical procedure for DASH based HTTP streaming includes the following steps:
         1) A DASH client obtains the MPD of a streaming content, e.g., a movie. The MPD includes information on different alternative representations, e.g., bit rate, video resolution, frame rate, audio language, of the streaming content, as well as URLs of the HTTP resources (the initialization segment and the media segments).   2) Based on information in the MPD and local information available to the DASH client, e.g., network bandwidth, decoding/display capabilities, and user preferences, the DASH client requests the desired representation(s), one segment (or a part thereof) at a time.   3) When the DASH client detects a network bandwidth change, it requests segments of a different representation with a better-matching bitrate, ideally starting from a segment that starts with a random access point.       

     During an HTTP streaming “session,” to respond to a user request to seek backward to a past position or forward to a future position, the DASH client requests past or future segments starting from a segment that is close to the desired position and that ideally starts with a random access point. The user may also request to fast-forward the content, which may be realized by requesting data sufficient for decoding only intra-coded video pictures or only a temporal subset of the video stream. 
     Video data may be encoded according to a variety of video coding standards. Such video coding standards include ITU-T H.261, ISO/IEC MPEG-1 Visual, ITU-T H.262 or ISO/IEC MPEG-2 Visual, ITU-T H.263, ISO/IEC MPEG-4 Visual, ITU-T H.264 or ISO/IEC MPEG-4 AVC, including its Scalable Video Coding (SVC) and Multiview Video Coding (MVC) extensions, and High-Efficiency Video Coding (HEVC), also known as ITU-T H.265 and ISO/IEC 23008-2, including its scalable coding extension (i.e., scalable high-efficiency video coding, SHVC) and multiview extension (i.e., multiview high efficiency video coding, MV-HEVC). 
     The OMAF draft specification describes various DASH descriptors for OMAF. Clause 8.2.1 of the OMAF draft specification specifies the projection format (PF) descriptor. Clause 8.2.2 of the OMAF draft specification specifies the region-wise packing (RWPK) descriptor. Clause 8.2.3 of the OMAF draft specification specifies the content coverage (CC) descriptor. Clause 8.2.4 of the OMAF draft specification specifies the region-wise quality ranking (RWQR) descriptor. 
       FIG.  1    is a block diagram illustrating an example system  10  that implements techniques for streaming media data over a network. In this example, system  10  includes content preparation device  20 , server device  60 , and client device  40 . Client device  40  and server device  60  are communicatively coupled by network  74 , which may comprise the Internet. In some examples, content preparation device  20  and server device  60  may also be coupled by network  74  or another network, or may be directly communicatively coupled. In some examples, content preparation device  20  and server device  60  may comprise the same device. 
     Content preparation device  20 , in the example of  FIG.  1   , comprises audio source  22  and video source  24 . Audio source  22  may comprise, for example, a microphone that produces electrical signals representative of captured audio data to be encoded by audio encoder  26 . Alternatively, audio source  22  may comprise a storage medium storing previously recorded audio data, an audio data generator such as a computerized synthesizer, or any other source of audio data. Video source  24  may comprise a video camera that produces video data to be encoded by video encoder  28 , a storage medium encoded with previously recorded video data, a video data generation unit such as a computer graphics source, or any other source of video data. Content preparation device  20  is not necessarily communicatively coupled to server device  60  in all examples, but may store multimedia content to a separate medium that is read by server device  60 . 
     Raw audio and video data may comprise analog or digital data. Analog data may be digitized before being encoded by audio encoder  26  and/or video encoder  28 . Audio source  22  may obtain audio data from a speaking participant while the speaking participant is speaking, and video source  24  may simultaneously obtain video data of the speaking participant. In other examples, audio source  22  may comprise a computer-readable storage medium comprising stored audio data, and video source  24  may comprise a computer-readable storage medium comprising stored video data. In this manner, the techniques described in this disclosure may be applied to live, streaming, real-time audio and video data or to archived, pre-recorded audio and video data. 
     Audio frames that correspond to video frames are generally audio frames containing audio data that was captured (or generated) by audio source  22  contemporaneously with video data captured (or generated) by video source  24  that is contained within the video frames. For example, while a speaking participant generally produces audio data by speaking, audio source  22  captures the audio data, and video source  24  captures video data of the speaking participant at the same time, that is, while audio source  22  is capturing the audio data. Hence, an audio frame may temporally correspond to one or more particular video frames. Accordingly, an audio frame corresponding to a video frame generally corresponds to a situation in which audio data and video data were captured at the same time and for which an audio frame and a video frame comprise, respectively, the audio data and the video data that was captured at the same time. 
     In some examples, audio encoder  26  may encode a timestamp in each encoded audio frame that represents a time at which the audio data for the encoded audio frame was recorded, and similarly, video encoder  28  may encode a timestamp in each encoded video frame that represents a time at which the video data for encoded video frame was recorded. In such examples, an audio frame corresponding to a video frame may comprise an audio frame comprising a timestamp and a video frame comprising the same timestamp. Content preparation device  20  may include an internal clock from which audio encoder  26  and/or video encoder  28  may generate the timestamps, or that audio source  22  and video source  24  may use to associate audio and video data, respectively, with a timestamp. 
     In some examples, audio source  22  may send data to audio encoder  26  corresponding to a time at which audio data was recorded, and video source  24  may send data to video encoder  28  corresponding to a time at which video data was recorded. In some examples, audio encoder  26  may encode a sequence identifier in encoded audio data to indicate a relative temporal ordering of encoded audio data but without necessarily indicating an absolute time at which the audio data was recorded, and similarly, video encoder  28  may also use sequence identifiers to indicate a relative temporal ordering of encoded video data. Similarly, in some examples, a sequence identifier may be mapped or otherwise correlated with a timestamp. 
     Audio encoder  26  generally produces a stream of encoded audio data, while video encoder  28  produces a stream of encoded video data. Each individual stream of data (whether audio or video) may be referred to as an elementary stream. An elementary stream is a single, digitally coded (possibly compressed) component of a representation. For example, the coded video or audio part of the representation can be an elementary stream. An elementary stream may be converted into a packetized elementary stream (PES) before being encapsulated within a video file. Within the same representation, a stream ID may be used to distinguish the PES-packets belonging to one elementary stream from the other. The basic unit of data of an elementary stream is a packetized elementary stream (PES) packet. Thus, coded video data generally corresponds to elementary video streams. Similarly, audio data corresponds to one or more respective elementary streams. 
     Many video coding standards, such as ITU-T H.264/AVC and the upcoming High Efficiency Video Coding (HEVC) standard, define the syntax, semantics, and decoding process for error-free bitstreams, any of which conform to a certain profile or level. Video coding standards typically do not specify the encoder, but the encoder is tasked with guaranteeing that the generated bitstreams are standard-compliant for a decoder. In the context of video coding standards, a “profile” corresponds to a subset of algorithms, features, or tools and constraints that apply to them. As defined by the H.264 standard, for example, a “profile” is a subset of the entire bitstream syntax that is specified by the H.264 standard. A “level” corresponds to the limitations of the decoder resource consumption, such as, for example, decoder memory and computation, which are related to the resolution of the pictures, bit rate, and block processing rate. A profile may be signaled with a profile_idc (profile indicator) value, while a level may be signaled with a level_idc (level indicator) value. 
     The H.264 standard, for example, recognizes that, within the bounds imposed by the syntax of a given profile, it is still possible to require a large variation in the performance of encoders and decoders depending upon the values taken by syntax elements in the bitstream such as the specified size of the decoded pictures. The H.264 standard further recognizes that, in many applications, it is neither practical nor economical to implement a decoder capable of dealing with all hypothetical uses of the syntax within a particular profile. Accordingly, the H.264 standard defines a “level” as a specified set of constraints imposed on values of the syntax elements in the bitstream. These constraints may be simple limits on values. Alternatively, these constraints may take the form of constraints on arithmetic combinations of values (e.g., picture width multiplied by picture height multiplied by number of pictures decoded per second). The H.264 standard further provides that individual implementations may support a different level for each supported profile. 
     A decoder conforming to a profile ordinarily supports all the features defined in the profile. For example, as a coding feature, B-picture coding is not supported in the baseline profile of H.264/AVC but is supported in other profiles of H.264/AVC. A decoder conforming to a level should be capable of decoding any bitstream that does not require resources beyond the limitations defined in the level. Definitions of profiles and levels may be helpful for interpretability. For example, during video transmission, a pair of profile and level definitions may be negotiated and agreed for a whole transmission session. More specifically, in H.264/AVC, a level may define limitations on the number of macroblocks that need to be processed, decoded picture buffer (DPB) size, coded picture buffer (CPB) size, vertical motion vector range, maximum number of motion vectors per two consecutive MBs, and whether a B-block can have sub-macroblock partitions less than 8×8 pixels. In this manner, a decoder may determine whether the decoder is capable of properly decoding the bitstream. 
     In the example of  FIG.  1   , encapsulation unit  30  of content preparation device  20  receives elementary streams comprising coded video data from video encoder  28  and elementary streams comprising coded audio data from audio encoder  26 . In some examples, video encoder  28  and audio encoder  26  may each include packetizers for forming PES packets from encoded data. In other examples, video encoder  28  and audio encoder  26  may each interface with respective packetizers for forming PES packets from encoded data. In still other examples, encapsulation unit  30  may include packetizers for forming PES packets from encoded audio and video data. 
     Video encoder  28  may encode video data of multimedia content in a variety of ways, to produce different representations of the multimedia content at various bitrates and with various characteristics, such as pixel resolutions, frame rates, conformance to various coding standards, conformance to various profiles and/or levels of profiles for various coding standards, representations having one or multiple views (e.g., for two-dimensional or three-dimensional playback), or other such characteristics. A representation, as used in this disclosure, may comprise one of audio data, video data, text data (e.g., for closed captions), or other such data. The representation may include an elementary stream, such as an audio elementary stream or a video elementary stream. Each PES packet may include a stream_id that identifies the elementary stream to which the PES packet belongs. Encapsulation unit  30  is responsible for assembling elementary streams into video files (e.g., segments) of various representations. 
     Encapsulation unit  30  receives PES packets for elementary streams of a representation from audio encoder  26  and video encoder  28  and forms corresponding network abstraction layer (NAL) units from the PES packets. Coded video segments may be organized into NAL units, which provide a “network-friendly” video representation addressing applications such as video telephony, storage, broadcast, or streaming. NAL units can be categorized to Video Coding Layer (VCL) NAL units and non-VCL NAL units. VCL units may contain the core compression engine and may include block, macroblock, and/or slice level data. Other NAL units may be non-VCL NAL units. In some examples, a coded picture in one time instance, normally presented as a primary coded picture, may be contained in an access unit, which may include one or more NAL units. 
     Non-VCL NAL units may include parameter set NAL units and SEI NAL units, among others. Parameter sets may contain sequence-level header information (in sequence parameter sets (SPS)) and the infrequently changing picture-level header information (in picture parameter sets (PPS)). With parameter sets (e.g., PPS and SPS), infrequently changing information need not to be repeated for each sequence or picture, hence coding efficiency may be improved. Furthermore, the use of parameter sets may enable out-of-band transmission of the important header information, avoiding the need for redundant transmissions for error resilience. In out-of-band transmission examples, parameter set NAL units may be transmitted on a different channel than other NAL units, such as SEI NAL units. 
     Supplemental Enhancement Information (SEI) may contain information that is not necessary for decoding the coded pictures samples from VCL NAL units, but may assist in processes related to decoding, display, error resilience, and other purposes. SEI messages may be contained in non-VCL NAL units. SEI messages are the normative part of some standard specifications, and thus are not always mandatory for standard compliant decoder implementation. SEI messages may be sequence level SEI messages or picture level SEI messages. Some sequence level information may be contained in SEI messages, such as scalability information SEI messages in the example of SVC and view scalability information SEI messages in MVC. These example SEI messages may convey information on, e.g., extraction of operation points and characteristics of the operation points. In addition, encapsulation unit  30  may form a manifest file, such as a media presentation descriptor (MPD) that describes characteristics of the representations. Encapsulation unit  30  may format the MPD according to extensible markup language (XML). 
     Encapsulation unit  30  may provide data for one or more representations of multimedia content, along with the manifest file (e.g., the MPD) to output interface  32 . Output interface  32  may comprise a network interface or an interface for writing to a storage medium, such as a universal serial bus (USB) interface, a CD or DVD writer or burner, an interface to magnetic or flash storage media, or other interfaces for storing or transmitting media data. Encapsulation unit  30  may provide data of each of the representations of multimedia content to output interface  32 , which may send the data to server device  60  via network transmission or storage media. In the example of  FIG.  1   , server device  60  includes storage medium  62  that stores various multimedia contents  64 , each including a respective manifest file  66  and one or more representations  68 A- 68 N (representations  68 ). In some examples, output interface  32  may also send data directly to network  74 . 
     In some examples, representations  68  may be separated into adaptation sets. That is, various subsets of representations  68  may include respective common sets of characteristics, such as codec, profile and level, resolution, number of views, file format for segments, text type information that may identify a language or other characteristics of text to be displayed with the representation and/or audio data to be decoded and presented, e.g., by speakers, camera angle information that may describe a camera angle or real-world camera perspective of a scene for representations in the adaptation set, rating information that describes content suitability for particular audiences, or the like. 
     Manifest file  66  may include data indicative of the subsets of representations  68  corresponding to particular adaptation sets, as well as common characteristics for the adaptation sets. Manifest file  66  may also include data representative of individual characteristics, such as bitrates, for individual representations of adaptation sets. In this manner, an adaptation set may provide for simplified network bandwidth adaptation. Representations in an adaptation set may be indicated using child elements of an adaptation set element of manifest file  66 . 
     Server device  60  includes request processing unit  70  and network interface  72 . In some examples, server device  60  may include a plurality of network interfaces. Furthermore, any or all of the features of server device  60  may be implemented on other devices of a content delivery network, such as routers, bridges, proxy devices, switches, or other devices. In some examples, intermediate devices of a content delivery network may cache data of multimedia content  64 , and include components that conform substantially to those of server device  60 . In general, network interface  72  is configured to send and receive data via network  74 . 
     Request processing unit  70  is configured to receive network requests from client devices, such as client device  40 , for data of storage medium  62 . For example, request processing unit  70  may implement hypertext transfer protocol (HTTP) version 1.1, as described in RFC 2616, “Hypertext Transfer Protocol—HTTP/1.1,” by R. Fielding et al, Network Working Group, IETF, June 1999. That is, request processing unit  70  may be configured to receive HTTP GET or partial GET requests and provide data of multimedia content  64  in response to the requests. The requests may specify a segment of one of representations  68 , e.g., using a URL of the segment. In some examples, the requests may also specify one or more byte ranges of the segment, thus comprising partial GET requests. Request processing unit  70  may further be configured to service HTTP HEAD requests to provide header data of a segment of one of representations  68 . In any case, request processing unit  70  may be configured to process the requests to provide requested data to a requesting device, such as client device  40 . 
     Additionally or alternatively, request processing unit  70  may be configured to deliver media data via a broadcast or multicast protocol, such as eMBMS. Content preparation device  20  may create DASH segments and/or sub-segments in substantially the same way as described, but server device  60  may deliver these segments or sub-segments using eMBMS or another broadcast or multicast network transport protocol. For example, request processing unit  70  may be configured to receive a multicast group join request from client device  40 . That is, server device  60  may advertise an Internet protocol (IP) address associated with a multicast group to client devices, including client device  40 , associated with particular media content (e.g., a broadcast of a live event). Client device  40 , in turn, may submit a request to join the multicast group. This request may be propagated throughout network  74 , e.g., routers making up network  74 , such that the routers are caused to direct traffic destined for the IP address associated with the multicast group to subscribing client devices, such as client device  40 . 
     As illustrated in the example of  FIG.  1   , multimedia content  64  includes manifest file  66 , which may correspond to a media presentation description (MPD). Manifest file  66  may contain descriptions of different alternative representations  68  (e.g., video services with different qualities) and the description may include, e.g., codec information, a profile value, a level value, a bitrate, and other descriptive characteristics of representations  68 . Client device  40  may retrieve the MPD of a media presentation to determine how to access segments of representations  68 . 
     In particular, retrieval unit  52  may retrieve configuration data (not shown) of client device  40  to determine decoding capabilities of video decoder  48  and rendering capabilities of video output  44 . The configuration data may also include any or all of a language preference selected by a user of client device  40 , one or more camera perspectives corresponding to depth preferences set by the user of client device  40 , and/or a rating preference selected by the user of client device  40 . Retrieval unit  52  may comprise, for example, a web browser or a media client configured to submit HTTP GET and partial GET requests. Retrieval unit  52  may correspond to software instructions executed by one or more processors or processing units (not shown) of client device  40 . In some examples, all or portions of the functionality described with respect to retrieval unit  52  may be implemented in hardware, or a combination of hardware, software, and/or firmware, where requisite hardware may be provided to execute instructions for software or firmware. 
     Retrieval unit  52  may compare the decoding and rendering capabilities of client device  40  to characteristics of representations  68  indicated by information of manifest file  66 . Retrieval unit  52  may initially retrieve at least a portion of manifest file  66  to determine characteristics of representations  68 . For example, retrieval unit  52  may request a portion of manifest file  66  that describes characteristics of one or more adaptation sets. Retrieval unit  52  may select a subset of representations  68  (e.g., an adaptation set) having characteristics that can be satisfied by the coding and rendering capabilities of client device  40 . Retrieval unit  52  may then determine bitrates for representations in the adaptation set, determine a currently available amount of network bandwidth, and retrieve segments from one of the representations having a bitrate that can be satisfied by the network bandwidth. 
     In general, higher bitrate representations may yield higher quality video playback, while lower bitrate representations may provide sufficient quality video playback when available network bandwidth decreases. Accordingly, when available network bandwidth is relatively high, retrieval unit  52  may retrieve data from relatively high bitrate representations, whereas when available network bandwidth is low, retrieval unit  52  may retrieve data from relatively low bitrate representations. In this manner, client device  40  may stream multimedia data over network  74  while also adapting to changing network bandwidth availability of network  74 . 
     Additionally or alternatively, retrieval unit  52  may be configured to receive data in accordance with a broadcast or multicast network protocol, such as eMBMS or IP multicast. In such examples, retrieval unit  52  may submit a request to join a multicast network group associated with particular media content. After joining the multicast group, retrieval unit  52  may receive data of the multicast group without further requests issued to server device  60  or content preparation device  20 . Retrieval unit  52  may submit a request to leave the multicast group when data of the multicast group is no longer needed, e.g., to stop playback or to change channels to a different multicast group. 
     Network interface  54  may receive and provide data of segments of a selected representation to retrieval unit  52 , which may in turn provide the segments to decapsulation unit  50 . Decapsulation unit  50  may decapsulate elements of a video file into constituent PES streams, depacketize the PES streams to retrieve encoded data, and send the encoded data to either audio decoder  46  or video decoder  48 , depending on whether the encoded data is part of an audio or video stream, e.g., as indicated by PES packet headers of the stream. Audio decoder  46  decodes encoded audio data and sends the decoded audio data to audio output  42 , while video decoder  48  decodes encoded video data and sends the decoded video data, which may include a plurality of views of a stream, to video output  44 . 
     Video encoder  28 , video decoder  48 , audio encoder  26 , audio decoder  46 , encapsulation unit  30 , retrieval unit  52 , and decapsulation unit  50  each may be implemented as any of a variety of suitable processing 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 encoder  28  and video decoder  48  may be included in one or more encoders or decoders, either of which may be integrated as part of a combined video encoder/decoder (CODEC). Likewise, each of audio encoder  26  and audio decoder  46  may be included in one or more encoders or decoders, either of which may be integrated as part of a combined CODEC. An apparatus including video encoder  28 , video decoder  48 , audio encoder  26 , audio decoder  46 , encapsulation unit  30 , retrieval unit  52 , and/or decapsulation unit  50  may comprise an integrated circuit, a microprocessor, and/or a wireless communication device, such as a cellular telephone. 
     Client device  40 , server device  60 , and/or content preparation device  20  may be configured to operate in accordance with the techniques of this disclosure. For purposes of example, this disclosure describes these techniques with respect to client device  40  and server device  60 . However, it should be understood that content preparation device  20  may be configured to perform these techniques, instead of (or in addition to) server device  60 . 
     Encapsulation unit  30  may form NAL units comprising a header that identifies a program to which the NAL unit belongs, as well as a payload, e.g., audio data, video data, or data that describes the transport or program stream to which the NAL unit corresponds. For example, in H.264/AVC, a NAL unit includes a 1-byte header and a payload of varying size. A NAL unit including video data in its payload may comprise various granularity levels of video data. For example, a NAL unit may comprise a block of video data, a plurality of blocks, a slice of video data, or an entire picture of video data. Encapsulation unit  30  may receive encoded video data from video encoder  28  in the form of PES packets of elementary streams. Encapsulation unit  30  may associate each elementary stream with a corresponding program. 
     Encapsulation unit  30  may also assemble access units from a plurality of NAL units. In general, an access unit may comprise one or more NAL units for representing a frame of video data, as well audio data corresponding to the frame when such audio data is available. An access unit generally includes all NAL units for one output time instance, e.g., all audio and video data for one time instance. For example, if each view has a frame rate of 20 frames per second (fps), then each time instance may correspond to a time interval of 0.05 seconds. During this time interval, the specific frames for all views of the same access unit (the same time instance) may be rendered simultaneously. In one example, an access unit may comprise a coded picture in one time instance, which may be presented as a primary coded picture. 
     Accordingly, an access unit may comprise all audio and video frames of a common temporal instance, e.g., all views corresponding to time X. This disclosure also refers to an encoded picture of a particular view as a “view component.” That is, a view component may comprise an encoded picture (or frame) for a particular view at a particular time. Accordingly, an access unit may be defined as comprising all view components of a common temporal instance. The decoding order of access units need not necessarily be the same as the output or display order. 
     A media presentation may include a media presentation description (MPD), which may contain descriptions of different alternative representations (e.g., video services with different qualities) and the description may include, e.g., codec information, a profile value, and a level value. An MPD is one example of a manifest file, such as manifest file  66 . Client device  40  may retrieve the MPD of a media presentation to determine how to access movie fragments of various presentations. Movie fragments may be located in movie fragment boxes (moof boxes) of video files. 
     Manifest file  66  (which may comprise, for example, an MPD) may advertise availability of segments of representations  68 . That is, the MPD may include information indicating the wall-clock time at which a first segment of one of representations  68  becomes available, as well as information indicating the durations of segments within representations  68 . In this manner, retrieval unit  52  of client device  40  may determine when each segment is available, based on the starting time as well as the durations of the segments preceding a particular segment. 
     After encapsulation unit  30  has assembled NAL units and/or access units into a video file based on received data, encapsulation unit  30  passes the video file to output interface  32  for output. In some examples, encapsulation unit  30  may store the video file locally or send the video file to a remote server via output interface  32 , rather than sending the video file directly to client device  40 . Output interface  32  may comprise, for example, a transmitter, a transceiver, a device for writing data to a computer-readable medium such as, for example, an optical drive, a magnetic media drive (e.g., floppy drive), a universal serial bus (USB) port, a network interface, or other output interface. Output interface  32  outputs the video file to a computer-readable medium, such as, for example, a transmission signal, a magnetic medium, an optical medium, a memory, a flash drive, or other computer-readable medium. 
     Network interface  54  may receive a NAL unit or access unit via network  74  and provide the NAL unit or access unit to decapsulation unit  50 , via retrieval unit  52 . Decapsulation unit  50  may decapsulate elements of a video file into constituent PES streams, depacketize the PES streams to retrieve encoded data, and send the encoded data to either audio decoder  46  or video decoder  48 , depending on whether the encoded data is part of an audio or video stream, e.g., as indicated by PES packet headers of the stream. Audio decoder  46  decodes encoded audio data and sends the decoded audio data to audio output  42 , while video decoder  48  decodes encoded video data and sends the decoded video data, which may include a plurality of views of a stream, to video output  44 . 
     In accordance with the techniques of this disclosure, content preparation device  20  (or server device  60 ) may construct data for manifest file  66  (e.g., a DASH MPD) representative of file level information of a track of a media file, such as a track of a segment of one of representations  68 . The data representative of the file level information may represent data of a box of the media file. The data may represent, for example, a four character code (4CC) of the box (also referred to as a code point), a version number of the box, and content of the box. The data may represent all of the data of the box or only a relevant portion of the box. 
     In some examples, encapsulation unit  30  of content preparation device  20  may automatically generate a descriptor of manifest file  66  from data of the box of the media file. Such descriptor may be an essential descriptor (e.g., if the track includes data other than a main media component) or a supplemental descriptor (e.g., if the track includes the main media component). Encapsulation unit  30  may generate a prefix such as “urn:mpeg:isobmff-dash:&lt;4cc&gt;:&lt;version&gt;,” preceding a value. The value of &lt;4cc&gt; may represent the four character code of the box, the value of &lt;version&gt; may represent the version of the box, and the following value may include data of the box itself, that is, all or a relevant portion of content of the box. 
     In this example, retrieval unit  52  may determine how to parse the value following the prefix based on the values of &lt;4cc&gt; and &lt;version&gt;. Retrieval unit  52  may then determine properties of the track using the descriptor, in particular, the content of the box represented in the value following the prefix. Retrieval unit  52  may further determine whether to select the track (and in particular, one of representations  68  or an adaptation set including one or more of representations  68  corresponding to one or more tracks including the track) based on the value following the prefix. For example, retrieval unit  52  may determine whether client device  40  is capable of decoding and rendering media data of the track, prioritize representations  68  (and tracks thereof), receive user input representing user preferences and select from among available representations  68  (and tracks thereof), or other such selection actions according to the value following the prefix. 
     Additionally or alternatively, encapsulation unit  30  may construct an element of manifest file  66  that can be used as an extension namespace in manifest file  66 . Encapsulation unit  30  may generate the element such that all relevant parameters of the box can be document in an XML descriptor. For example, encapsulation unit  30  may use elements, attributes, mandatory and optional parameters, proper data types, or the like. Retrieval unit  52  may use the element and the extension namespace in a manner similar to the descriptor as discussed above, e.g., to select a track (and corresponding one of representations  68  or an adaptation set including one or more representations  68 ). 
     As yet another example, encapsulation unit  30  may automatically generate a descriptor as discussed above, but follow an automated syntax for expressing all data in the box, instead of a single value field. Encapsulation unit  30  may automatically generate XML or Augmented Backus-Naur Form (ABNF) formatted data for the descriptor. As an example, encapsulation unit  30  may construct a descriptor comprising a prefix “urn:mpeg:isobmff-dash:processing,” preceding a value, where the value defines an extension namespace identifier. Retrieval unit  52  may use the descriptor in a manner similar to the descriptor or element as discussed above, e.g., to select a track (and corresponding one of representations  68  or an adaptation set including one or more representations  68 ). 
     In this manner, content preparation device  20  and/or server device  60  represent examples of a device for generating media data, the device comprising a memory configured to store media data; and a processor implemented in circuitry and configured to process a box of file level information that describes a property of a track of a media file of media content including the media data, generate a descriptor for a manifest file of the media content, the descriptor corresponding to an adaptation set of the media content and including data representative of the box of the file level information, the media data of the track being included in the adaptation set, and send the manifest file including the descriptor to a client device. 
     Likewise, client device  40  represents an example of a device for retrieving media data including a memory configured to store media data; and a processor implemented in circuitry and configured to: process a descriptor of a manifest file of media content including the media data, the descriptor corresponding to an adaptation set of the media content and including data representative of a box of file level information that describes a property of a track of a media file of the media content including media data of the adaptation set, determine whether to retrieve the media data of the adaptation set according to the data representative of the file level information, and, in response to determining to retrieve the media data, send a request to retrieve the media data. 
       FIG.  2    is a block diagram illustrating an example set of components of retrieval unit  52  of  FIG.  1    in greater detail. In this example, retrieval unit  52  includes eMBMS middleware unit  100 , DASH client  110 , and media application  112 . 
     In this example, eMBMS middleware unit  100  further includes eMBMS reception unit  106 , cache  104 , and proxy server  102 . In this example, eMBMS reception unit  106  is configured to receive data via eMBMS, e.g., according to File Delivery over Unidirectional Transport (FLUTE), described in T. Paila et al., “FLUTE—File Delivery over Unidirectional Transport,” Network Working Group, RFC 6726, November 2012, available at http://tools.ietf.org/html/rfc6726. That is, eMBMS reception unit  106  may receive files via broadcast from, e.g., server device  60 , which may act as a BM-SC. 
     As eMBMS middleware unit  100  receives data for files, eMBMS middleware unit may store the received data in cache  104 . Cache  104  may comprise a computer-readable storage medium, such as flash memory, a hard disk, RAM, or any other suitable storage medium. 
     Proxy server  102  may act as a server for DASH client  110 . For example, proxy server  102  may provide a MPD file or other manifest file to DASH client  110 . Proxy server  102  may advertise availability times for segments in the MPD file, as well as hyperlinks from which the segments can be retrieved. These hyperlinks may include a localhost address prefix corresponding to client device  40  (e.g., 127.0.0.1 for IPv4). In this manner, DASH client  110  may request segments from proxy server  102  using HTTP GET or partial GET requests. For example, for a segment available from link http://127.0.0.1/rep1/seg3, DASH client  110  may construct an HTTP GET request that includes a request for http://127.0.0.1/rep1/seg3, and submit the request to proxy server  102 . Proxy server  102  may retrieve requested data from cache  104  and provide the data to DASH client  110  in response to such requests. 
     DASH client  110  may be configured according to any or all of the techniques of this disclosure as discussed above, alone or in any combination. 
       FIG.  3    is a conceptual diagram illustrating elements of example multimedia content  120 . Multimedia content  120  may correspond to multimedia content  64  ( FIG.  1   ), or another multimedia content stored in storage medium  62 . In the example of  FIG.  3   , multimedia content  120  includes media presentation description (MPD)  122  and a plurality of representations  124 A- 124 N (representations  124 ). Representation  124 A includes optional header data  126  and segments  128 A- 128 N (segments  128 ), while representation  124 N includes optional header data  130  and segments  132 A- 132 N (segments  132 ). The letter N is used to designate the last movie fragment in each of representations  124  as a matter of convenience. In some examples, there may be different numbers of movie fragments between representations  124 . 
     MPD  122  may comprise a data structure separate from representations  124 . MPD  122  may correspond to manifest file  66  of  FIG.  1   . Likewise, representations  124  may correspond to representations  68  of  FIG.  2   . In general, MPD  122  may include data that generally describes characteristics of representations  124 , such as coding and rendering characteristics, adaptation sets, a profile to which MPD  122  corresponds, text type information, camera angle information, rating information, trick mode information (e.g., information indicative of representations that include temporal sub-sequences), and/or information for retrieving remote periods (e.g., for targeted advertisement insertion into media content during playback). 
     Header data  126 , when present, may describe characteristics of segments  128 , e.g., temporal locations of random access points (RAPs, also referred to as stream access points (SAPs)), which of segments  128  includes random access points, byte offsets to random access points within segments  128 , uniform resource locators (URLs) of segments  128 , or other aspects of segments  128 . Header data  130 , when present, may describe similar characteristics for segments  132 . Additionally or alternatively, such characteristics may be fully included within MPD  122 . 
     Segments  128 ,  132  include one or more coded video samples, each of which may include frames or slices of video data. Each of the coded video samples of segments  128  may have similar characteristics, e.g., height, width, and bandwidth requirements. Such characteristics may be described by data of MPD  122 , though such data is not illustrated in the example of  FIG.  3   . MPD  122  may include characteristics as described by the 3GPP Specification, with the addition of any or all of the signaled information described in this disclosure. 
     Each of segments  128 ,  132  may be associated with a unique uniform resource locator (URL). Thus, each of segments  128 ,  132  may be independently retrievable using a streaming network protocol, such as DASH. In this manner, a destination device, such as client device  40 , may use an HTTP GET request to retrieve segments  128  or  132 . In some examples, client device  40  may use HTTP partial GET requests to retrieve specific byte ranges of segments  128  or  132 . 
     MPD  122  may include data constructed according to any or all of the techniques of this disclosure, alone or in any combination. 
       FIG.  4    is a block diagram illustrating elements of an example video file  150 , which may correspond to a segment of a representation, such as one of segments  114 ,  124  of  FIG.  3   . Each of segments  128 ,  132  may include data that conforms substantially to the arrangement of data illustrated in the example of  FIG.  4   . Video file  150  may be said to encapsulate a segment. As described above, video files in accordance with the ISO base media file format and extensions thereof store data in a series of objects, referred to as “boxes.” In the example of  FIG.  4   , video file  150  includes file type (FTYP) box  152 , movie (MOOV) box  154 , segment index (sidx) boxes  162 , movie fragment (MOOF) boxes  164 , and movie fragment random access (MFRA) box  166 . Although  FIG.  4    represents an example of a video file, it should be understood that other media files may include other types of media data (e.g., audio data, timed text data, or the like) that is structured similarly to the data of video file  150 , in accordance with the ISO base media file format and its extensions. 
     File type (FTYP) box  152  generally describes a file type for video file  150 . File type box  152  may include data that identifies a specification that describes a best use for video file  150 . File type box  152  may alternatively be placed before MOOV box  154 , movie fragment boxes  164 , and/or MFRA box  166 . 
     In some examples, a Segment, such as video file  150 , may include an MPD update box (not shown) before FTYP box  152 . The MPD update box may include information indicating that an MPD corresponding to a representation including video file  150  is to be updated, along with information for updating the MPD. For example, the MPD update box may provide a URI or URL for a resource to be used to update the MPD. As another example, the MPD update box may include data for updating the MPD. In some examples, the MPD update box may immediately follow a segment type (STYP) box (not shown) of video file  150 , where the STYP box may define a segment type for video file  150 .  FIG.  7   , discussed in greater detail below, provides additional information with respect to the MPD update box. 
     MOOV box  154 , in the example of  FIG.  4   , includes movie header (MVHD) box  156 , track (TRAK) box  158 , and one or more movie extends (MVEX) boxes  160 . In general, MVHD box  156  may describe general characteristics of video file  150 . For example, MVHD box  156  may include data that describes when video file  150  was originally created, when video file  150  was last modified, a timescale for video file  150 , a duration of playback for video file  150 , or other data that generally describes video file  150 . 
     TRAK box  158  may include data for a track of video file  150 . TRAK box  158  may include a track header (TKHD) box that describes characteristics of the track corresponding to TRAK box  158 . In some examples, TRAK box  158  may include coded video pictures, while in other examples, the coded video pictures of the track may be included in movie fragments  164 , which may be referenced by data of TRAK box  158  and/or sidx boxes  162 . Moreover, in accordance with the techniques of this disclosure, a manifest file (such as an MPD) may include data representing data of TRAK box  158 , albeit separately from video file  150 . In this manner, client device  40  ( FIG.  1   ) may avoid retrieving video file  150  initially and determine whether to retrieve video file  150  (or a portion thereof, such as a particular track of video file  150 ) using the data of the manifest file representative of TRAK box  158 . 
     In some examples, video file  150  may include more than one track. Accordingly, MOOV box  154  may include a number of TRAK boxes equal to the number of tracks in video file  150 . TRAK box  158  may describe characteristics of a corresponding track of video file  150 . For example, TRAK box  158  may describe temporal and/or spatial information for the corresponding track. A TRAK box similar to TRAK box  158  of MOOV box  154  may describe characteristics of a parameter set track, when encapsulation unit  30  ( FIG.  3   ) includes a parameter set track in a video file, such as video file  150 . Encapsulation unit  30  may signal the presence of sequence level SEI messages in the parameter set track within the TRAK box describing the parameter set track. 
     MVEX boxes  160  may describe characteristics of corresponding movie fragments  164 , e.g., to signal that video file  150  includes movie fragments  164 , in addition to video data included within MOOV box  154 , if any. In the context of streaming video data, coded video pictures may be included in movie fragments  164  rather than in MOOV box  154 . Accordingly, all coded video samples may be included in movie fragments  164 , rather than in MOOV box  154 . 
     MOOV box  154  may include a number of MVEX boxes  160  equal to the number of movie fragments  164  in video file  150 . Each of MVEX boxes  160  may describe characteristics of a corresponding one of movie fragments  164 . For example, each MVEX box may include a movie extends header box (MEHD) box that describes a temporal duration for the corresponding one of movie fragments  164 . 
     As noted above, encapsulation unit  30  may store a sequence data set in a video sample that does not include actual coded video data. A video sample may generally correspond to an access unit, which is a representation of a coded picture at a specific time instance. In the context of AVC, the coded picture includes one or more VCL NAL units which contains the information to construct all the pixels of the access unit and other associated non-VCL NAL units, such as SEI messages. Accordingly, encapsulation unit  30  may include a sequence data set, which may include sequence level SEI messages, in one of movie fragments  164 . Encapsulation unit  30  may further signal the presence of a sequence data set and/or sequence level SEI messages as being present in one of movie fragments  164  within the one of MVEX boxes  160  corresponding to the one of movie fragments  164 . 
     SIDX boxes  162  are optional elements of video file  150 . That is, video files conforming to the 3GPP file format, or other such file formats, do not necessarily include SIDX boxes  162 . In accordance with the example of the 3GPP file format, a SIDX box may be used to identify a sub-segment of a segment (e.g., a segment contained within video file  150 ). The 3GPP file format defines a sub-segment as “a self-contained set of one or more consecutive movie fragment boxes with corresponding Media Data box(es) and a Media Data Box containing data referenced by a Movie Fragment Box must follow that Movie Fragment box and precede the next Movie Fragment box containing information about the same track.” The 3GPP file format also indicates that a SIDX box “contains a sequence of references to subsegments of the (sub)segment documented by the box. The referenced subsegments are contiguous in presentation time. Similarly, the bytes referred to by a Segment Index box are always contiguous within the segment. The referenced size gives the count of the number of bytes in the material referenced.” 
     SIDX boxes  162  generally provide information representative of one or more sub-segments of a segment included in video file  150 . For instance, such information may include playback times at which sub-segments begin and/or end, byte offsets for the sub-segments, whether the sub-segments include (e.g., start with) a stream access point (SAP), a type for the SAP (e.g., whether the SAP is an instantaneous decoder refresh (IDR) picture, a clean random access (CRA) picture, a broken link access (BLA) picture, or the like), a position of the SAP (in terms of playback time and/or byte offset) in the sub-segment, and the like. 
     Movie fragments  164  may include one or more coded video pictures. In some examples, movie fragments  164  may include one or more groups of pictures (GOPs), each of which may include a number of coded video pictures, e.g., frames or pictures. In addition, as described above, movie fragments  164  may include sequence data sets in some examples. Each of movie fragments  164  may include a movie fragment header box (MFHD, not shown in  FIG.  4   ). The MFHD box may describe characteristics of the corresponding movie fragment, such as a sequence number for the movie fragment. Movie fragments  164  may be included in order of sequence number in video file  150 . 
     MFRA box  166  may describe random access points within movie fragments  164  of video file  150 . This may assist with performing trick modes, such as performing seeks to particular temporal locations (i.e., playback times) within a segment encapsulated by video file  150 . MFRA box  166  is generally optional and need not be included in video files, in some examples. Likewise, a client device, such as client device  40 , does not necessarily need to reference MFRA box  166  to correctly decode and display video data of video file  150 . MFRA box  166  may include a number of track fragment random access (TFRA) boxes (not shown) equal to the number of tracks of video file  150 , or in some examples, equal to the number of media tracks (e.g., non-hint tracks) of video file  150 . 
     In some examples, movie fragments  164  may include one or more stream access points (SAPs), such as IDR pictures. Likewise, MFRA box  166  may provide indications of locations within video file  150  of the SAPs. Accordingly, a temporal sub-sequence of video file  150  may be formed from SAPs of video file  150 . The temporal sub-sequence may also include other pictures, such as P-frames and/or B-frames that depend from SAPs. Frames and/or slices of the temporal sub-sequence may be arranged within the segments such that frames/slices of the temporal sub-sequence that depend on other frames/slices of the sub-sequence can be properly decoded. For example, in the hierarchical arrangement of data, data used for prediction for other data may also be included in the temporal sub-sequence. 
     Video file  150  may include data constructed according to any or all of the techniques of this disclosure, alone or in any combination. 
       FIG.  5    is a conceptual diagram illustrating an example content model  180  for DASH multitrack. In the DASH Technologies under Consideration (TuC), the model on content annotation and decisions has been updated to address the selection of content. In order to support the content author in providing content in a consistent manner,  FIG.  5    provides a conceptual content model for DASH content in one Period of an MPD. The content may be described by an Asset Identifier as a whole and may contain different media types, video, audio, subtitle and application types. 
     In particular, content model  180  includes content  182 , described as a whole by asset description  204 . Content  182  may include multiple, various media types  206 , such as media type video  184 , media type audio  186 , media type subtitle  188  (also referred to as timed text), and/or media type application  190 . The various types of media content may be provided in sets of media types, such as media type content main  192 , media type content alternative (alt)  1   194 , and media type content alt  2   196 . A media application may select one of these sets of media types, such that the collection of sets of media types may be referred to as application-based selection media  208 . These sets may further correspond to target version adaptation sets  200 A- 200 C (target version adaptation sets  200 ), which client device  40  may select through an automated system-based selection process  210 . Target version adaptation sets  200  may each include respective encoded representations  202 A- 202 C of various bitrates to provide for dynamic switching  212 , e.g., as available bandwidth increases or decreases. 
     Within each of multiple media types  206 , the content author may want to offer different alternative content that are time-aligned (e.g., media type content main, media type content alt  1   194 , and media type content alt  2   196 ), but each alternative represents different content. Automatic selection of the alternative content is not expected to be done by the DASH client as the DASH client would not have sufficient information to make such decisions. However, the selection is expected to be done by communication with an application or the user, typically using a user interface appropriate for selection. 
     In the absence of this external communication, or at startup, the DASH client still needs to playback content, and therefore benefits from information indicative of default content. Such signalling should be provided by the content author. Such default content may be referred to as main content (e.g., media type content main  192 ), whereas any content that is not main may be referred to as alternative (e.g., media type content alt  1   194 , media type content alt  2   196 ). There may be multiple alternatives which may need to be distinguished. This disclosure defines main and alternative content. Examples for such are synchronized camera views of one master content. The main camera view is provided as main content, all other views as alternative content. 
     Furthermore, it may be that content of different media types are linked by the content author, to express that two contents of different media types are preferably played together. This disclosure defines associated contents for this purpose. As an example, there may be a main commentator associated with the main camera view, but for a different camera view, a different associated commentary is provided. 
     In addition to semantical content level differentiation, each alternative content may be prepared with different target versions, based on content preparation properties (downmix, subsampling, translation, suitable for trick mode, etc.), client preferences (decoding or rendering preferences, e.g. codec), client capabilities (DASH profile support, decoding capabilities, rendering capabilities) or user preferences (accessibility, language, etc.). In simple AV playout and in the absence of guidance from an application, a content author expects that the DASH client selects at most one target version for each Group (e.g., one of target version adaptation sets  200 ), taking into account its capabilities and preferences and the capabilities and preferences of the media subsystem. However, an application may obviously select multiple Groups and playout different video Adaptation Sets to support for example picture-in-picture, multi-angle and so on. 
     In addition, the content author may also provide priorities for target versions, if the receivers support selection of content according to multiple priorities. Typical examples are that the content is prepared for H.264/AVC and H.265/HEVC capable receivers, and the content author prefers the selection of the H.265/HEVC version as its distribution is more efficient. A device supporting both decoders may then choose the one with higher priority signalled by the content author. In a similar version, the same content may be provided in different languages. In this case, it can still be expected that the language can be automatically selected by the client, so it is assigned to a target version. Again, a content author may express priorities on languages, for example preferring the native language over a dubbed one. Languages may be considered as alternative content as well, but as long as automatic selection can be provided, it may be considered as different target versions. Hence for each content of one media type, different target versions may exist and the annotation of the content expressed that it is expected that automated selection can be done. Each target version is preferably accumulated in one Adaptation Set, with exceptions such as scalable codecs. 
     Finally, in the content model, each of the target version typically has multiple encoded representations  202  that are prepared to enable dynamic switching. This aspect is outside the scope of this section as switching by the client is expected to be done independent of the media type as well as the target version, primarily using the bandwidth and possibly abstract quality information. However, the signalling on the target versions may provide information on how to distribute the available bitrate across different media types. 
     Based on this content model and the available elements, attributes and descriptors in DASH, the TuC of DASH provides requirements and recommendations for Adaptation Set Signalling to address main and alternative content, associated content as well as different target versions. Based on the signalling, a client decision model is developed that may serve a content provider as a reference client to test if the annotation provided in the MPD provides the proper results. 
       FIG.  6    is a conceptual diagram illustrating an example client model  220 . In particular, client model  220  includes DASH client  232 , media application  238 , file format processing unit  240 , media decoder  242 , media renderer  244 , and output device  246 . DASH client  232  and media application  238  may generally correspond to retrieval unit  52  of  FIG.  1   , file format processing unit  240  may correspond to decapsulation unit  50  of  FIG.  1   , and media decoder  242  may correspond to either or both of audio decoder  46  and video decoder  48 , and media renderer  244  and output device  246  may correspond to either or both of audio output  42  and video output  44  of  FIG.  1   . 
     In this example, each of file formats  222 A- 222 C (file formats  222 ) includes respective representations  224 A,  224 B,  226 A,  226 B,  228 A,  228 B, described by MPD  230  (an example of a manifest file). MPD  230  includes data according to the techniques of this disclosure describing properties of tracks of media files formatted according to one of file formats  222 . Thus, DASH client  232  initially retrieves MPD  230 . DASH client  232  receives requested media data (e.g., user preferences, priorities, device capabilities, or the like) from media application  238 , in this example. DASH client  232  processes the data of MPD  230  to determine properties of tracks of file formats  222 , to cause selection unit  234  to select an appropriate one of file formats  222  for, e.g., file format processing unit  240  and conforming to user preferences, priorities, and the like. Download &amp; switching unit  236  of DASH client  232  then retrieves media data of one of representations  224 A,  224 B,  226 A,  226 B,  228 A, or  228 B, according to which of file formats  222  was selected and an available amount of network bandwidth. DASH client  230  provides the retrieved media data to file format processing unit  240 , which decapsulates the media data and provides the media data to media application  238 . Media application  238 , in turn, provides the media data to media decoder  242 , which decodes the media data and passes the decoded media data to media renderer  244 . Media renderer  244 , in turn, renders the media data and provides the rendered media data to output device  246  for output (e.g., audio or video output). 
     With reference to the terms “main content,” “associated content,” and “target versions” above, DASH client  232  may operate selection unit  234  that is configured to perform selection according to communication with media application  238  in  FIG.  6   . This selection is based on information in MPD  230 , assigned to each Adaptation Set (or Preselection, if DASH Amd. 4 is in place) of file formats  222 . A content preparation device, such as content preparation device  20  of  FIG.  1   , adds sufficient additional metadata to MPD  230  in order to enable the selection, in accordance with the techniques of this disclosure. Typically, DASH client  232  extracts this information, which is aligned with information that is also present in the ISO BMFF track, primarily because of the track metadata provide sufficient information to describe the track. 
     In even another option, file formats  222  (and potentially also application formats, such as OMAF) define sufficient information for description of the tracks for selection. The generic file format processor typically also communicates with the application in order to properly make use of the track and to possibly render the track. 
     The information in DASH and ISO Base Media File Format may be identical in expressing information for media application  238  for their selection. It may be confusing if the metadata in the file format is not aligned with the information in the file format. 
     Not necessarily all information needs to be expressed on DASH level for the purpose for selection, but sufficient information may be expressed in order to provide media application  238  and selection unit  234  the ability to properly differentiate and select between file formats  222 . 
     Conventionally, this is solved primarily by the file format defining specific metadata information, and on DASH level, some descriptors are defined that more less may match. However, this requires a completely new definition in DASH and unnecessarily may delay work, as the descriptors need to be done afterwards. The approach is also error-prone and typically not complete and comprehensive. It would be much more suitable that any metadata definition in ISO BMFF formats is automatically exposed to DASH level and the DASH client is aware that the information included in the DASH “descriptor” is aligned with file format definition. This also allows the same information to be interpreted in the same way by the application, independent of the source. 
     Conventionally, a descriptor is defined for each and every type of file format information that needs to be exposed to the DASH MPD, resulting in many complex descriptor definitions, and mismatches between the signaling in the MPD and in file format boxes can easy occur. 
     Referring back to  FIG.  1   , content preparation device  20 , server device  60 , and client device  40  may be configured according to the following basic design, per the techniques of this disclosure:
         1) For any file level information that describes the property of a track, the information can be exposed at the DASH level by an automatic conversion of the 4-character code (4CC), the version number, and the content of the box.   2) Either all information in a box is provided, or a part of all information that is relevant is provided.   3) The information can be used as a regular descriptor in DASH that can be used for the selection of Adaptation Sets and Preselections.       

     The mechanisms above may be used to replace all the descriptors, namely PF, RWPK, CC, and RWQR descriptors, defined in clause 8.2 of the OMAF draft text in m40849, or may be used in addition to these descriptors. 
     The exposition of such information may follow different design choices. Example design choices are described below: 
     Example option 0: add movie header. This is the simplest version and only provides the Movie Header in the MPD. This obviously contains relevant information, but results in detailed parsing, and key information may be lost. 
     Example option 1: generate automated descriptor. In this example, an automated descriptor is generated that can be used with the DASH Essential and Supplemental property descriptor. In this case, the regular descriptor mode in DASH can be used. The descriptor may be generated by a prefix such as urn:mpeg:isobmff-dash:&lt;4cc&gt;:&lt;version&gt;, and the value may be the content of the data in the box. One advantage of this approach is that it is simple, but the value field may be large, and non-essential data may possibly be added. This example also allows this method to be applied to existing features. 
     Example option 2: generate extensions namespace and XML. In this example, when defining a new box, an element is also created that can used as an extension namespace in the MPD. The element is generated consciously, such that all relevant parameters of the box can be documented in the XML descriptor as well, for example, by the use of elements and attributes, mandatory and optional parameters, proper data types, and the like. One advantage of this approach is that it is more powerful and readable, but the diligence of the design is important, and this does not apply for existing data. In order to indicate whether the processing of the metadata is supplemental or essential, a generic descriptor may be added that refers to the 4CC. 
     Example option 3: Mix of example options 1 and 2. In this example, the descriptor is generated automatically, but instead of using a single value field, an automated syntax for expressing all data in the box is provided. The syntax may, for example, be an automatically generated XML or an automatically generated Augmented Backus-Naur Form (ABNF). Reuse of existing functionalities is encouraged. 
     In one example, a projected omnidirectional video box may be added. Various options are described below for adding data to an Adaptation Set element. 
     Example Option 1: Add Movie Header 
     
         
         
           
             @movieHeader=“Xxxuxox” 
           
         
       
    
     Example Option 2: Add Descriptor 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 &lt;EssentialDescriptor 
               
               
                   
                    schemeUdURI=“urn:mpeg:isobmff-dash:povd:0” 
               
               
                   
                    value=“Xxxuxox” 
               
               
                   
                 /&gt; 
               
               
                   
                   
               
            
           
         
       
     
     Example Option 3: Extension Namespace 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 &lt;EssentialDescriptor 
               
               
                   
                    schemeUdURI=urn:mpeg:isobmff-dash:processing 
               
               
                   
                    value=“povd”/&gt; 
               
               
                   
                 &lt;isobmff:povd 
               
               
                   
                       projectionType=0/&gt; 
               
               
                   
                 /&gt; 
               
               
                   
                   
               
            
           
         
       
     
     Example Option 4: Automated Generation 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 &lt;EssentialDescriptor 
               
               
                   
                    schemeUdURI=urn:mpeg:isobmff-dash:processing 
               
               
                   
                    value=“povd”/&gt; 
               
               
                   
                 &lt;isobmff:povd 
               
               
                   
                    &lt;prfr 
               
               
                   
                       projection_type=0/&gt; 
               
               
                   
                 /&gt; 
               
               
                   
                   
               
            
           
         
       
     
       FIG.  7    is a flowchart illustrating an example method for generating media data and for retrieving media data in accordance with the techniques of this disclosure. The method of  FIG.  7    is explained with respect to content preparation device  20  and client device  40  of  FIG.  1   , although it should be understood that other devices may be configured to perform this or a similar method. For example, server device  60  may be configured to perform some or all of the elements attributed to content preparation device  20 . 
     Initially, content preparation device  20  obtains a media file ( 300 ). The media file may include audio data, video data, timed text data, or the like. For example, the media file may correspond to media content according to the model shown in  FIG.  5   . The media file may conform to ISO BMFF, e.g., as shown in  FIG.  4   . It is assumed that the media file includes one or more track boxes, such as TRAK box  158 , which describe properties of respective tracks of the media file. Thus, content preparation device  20  determines the boxes of the media file describing the tracks ( 302 ). 
     Content preparation device  20  further generates descriptors including data for the boxes ( 304 ), in this example. For example, the descriptor may be a single element or multiple elements of an extension namespace, as discussed above. The data for the box may include, for example, a four character code (4CC) of the box, a version of the box, and some or all of the content of the box. For example, the generated descriptors may correspond to any or all of example options 1-4 discussed above. 
     Content preparation device  20  may then add the descriptors to a manifest file ( 306 ), such as a DASH MPD, for the media content. Content preparation device  20  then sends the manifest file to client device  40  ( 308 ). Client device  40  receives the manifest file ( 310 ) and processes the descriptors of the manifest file ( 312 ) to select one or more adaptation sets using the descriptors ( 314 ). For example, as discussed above, client device  40  may select the adaptation sets according to user preferences, priorities, capabilities of client device  40  (e.g., decoding and rendering capabilities, file format processing capabilities, or the like), or other such selection criteria. 
     Ultimately, client device  40  may send a request for media data of a selected adaptation set ( 316 ) to content preparation device  20  (or server device  60 ). In particular, client device  40  may determine one of the representations of the adaptation set from which to retrieve media data of a track corresponding to the selected adaptation set, e.g., based on an available amount of network bandwidth and a bitrate of the representation. Content preparation device  20  (or server device  60 ) may receive the request ( 318 ) and send the requested media data to client device  40  ( 320 ). Ultimately, client device  40  may receive the media data ( 322 ) and send the media data to a media decoder ( 324 ) to be decoded, and ultimately rendered and presented. 
     In this manner, the method of  FIG.  7    represents an example of a method of generating media data including processing a box of file level information that describes a property of a track of a media file of media content including media data, generating a descriptor for a manifest file of the media content, the descriptor corresponding to an adaptation set of the media content and including data representative of the box of the file level information, the media data of the track being included in the adaptation set, and sending the manifest file including the descriptor to a client device. 
     The method of  FIG.  7    also represents an example of a method of retrieving media data including processing a descriptor of a manifest file of media content, the descriptor corresponding to an adaptation set of the media content and including data representative of a box of file level information that describes a property of a track of a media file of the media content including media data of the adaptation set, determining whether to retrieve the media data of the adaptation set according to the data representative of the file level information, and, in response to determining to retrieve the media data, sending a request to retrieve the media data. 
     In one or more examples, the functions described may be implemented in hardware, software, firmware, or any combination thereof. If implemented in software, the functions may be stored on or transmitted over as one or more instructions or code on a computer-readable medium and executed by a hardware-based processing unit. Computer-readable media may include computer-readable storage media, which corresponds to a tangible medium such as data storage media, or communication media including any medium that facilitates transfer of a computer program from one place to another, e.g., according to a communication protocol. In this manner, computer-readable media generally may correspond to (1) tangible computer-readable storage media which is non-transitory or (2) a communication medium such as a signal or carrier wave. Data storage media may be any available media that can be accessed by one or more computers or one or more processors to retrieve instructions, code, and/or data structures for implementation of the techniques described in this disclosure. A computer program product may include a computer-readable medium. 
     By way of example, and not limitation, such computer-readable storage media can comprise RAM, ROM, EEPROM, CD-ROM or other optical disk storage, magnetic disk storage, or other magnetic storage devices, flash memory, or any other medium that can be used to store desired program code in the form of instructions or data structures and that can be accessed by a computer. Also, any connection is properly termed a computer-readable medium. For example, if instructions are transmitted from a website, server, or other remote source using a coaxial cable, fiber optic cable, twisted pair, digital subscriber line (DSL), or wireless technologies such as infrared, radio, and microwave, then the coaxial cable, fiber optic cable, twisted pair, DSL, or wireless technologies such as infrared, radio, and microwave are included in the definition of medium. It should be understood, however, that computer-readable storage media and data storage media do not include connections, carrier waves, signals, or other transitory media, but are instead directed to non-transitory, tangible storage media. Disk and disc, as used herein, includes compact disc (CD), laser disc, optical disc, digital versatile disc (DVD), floppy disk and Blu-ray disc where disks usually reproduce data magnetically, while discs reproduce data optically with lasers. Combinations of the above should also be included within the scope of computer-readable media. 
     Instructions may be executed by one or more processors, such as one or more digital signal processors (DSPs), general purpose microprocessors, application specific integrated circuits (ASICs), field programmable logic arrays (FPGAs), or other equivalent integrated or discrete logic circuitry. Accordingly, the term “processor,” as used herein may refer to any of the foregoing structure or any other structure suitable for implementation of the techniques described herein. In addition, in some aspects, the functionality described herein may be provided within dedicated hardware and/or software modules configured for encoding and decoding, or incorporated in a combined codec. Also, the techniques could be fully implemented in one or more circuits or logic elements. 
     The techniques of this disclosure may be implemented in a wide variety of devices or apparatuses, including a wireless handset, an integrated circuit (IC) or a set of ICs (e.g., a chip set). Various components, modules, or units are described in this disclosure to emphasize functional aspects of devices configured to perform the disclosed techniques, but do not necessarily require realization by different hardware units. Rather, as described above, various units may be combined in a codec hardware unit or provided by a collection of interoperative hardware units, including one or more processors as described above, in conjunction with suitable software and/or firmware. 
     Various examples have been described. These and other examples are within the scope of the following claims.