Abstract:
A method and a device for encapsulating a media entity containing more than one layer into multiple component files, each for one layer, are described along with the corresponding method and device for component file reading. Extensions to the Extractor data structure of SVC/MVC file formats are proposed. The extractor extensions of the invention enable NAL units referencing across different component files. The present invention enables adaptive HTTP streaming of media entities.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application for patent claims the benefit of priority from U.S. Provisional Patent Application Ser. No. 61/354,422, entitled “Extension to the Extractor data structure of SVC/MVC file formats,” and filed on Jun. 14, 2010. The teachings of the above-identified provisional patent application are expressly incorporated herein by reference. 
         [0002]    The present application is related to the following co-pending, commonly owned U.S. patent application Ser. No. ______/______ entitled “Method and Apparatus for Encapsulating Coded Multi-component Video”, filed concurrently herewith (Attorney Docket No. PU100141). The teachings of the non-provisional patent applications identified immediately above are expressly incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0003]    The present invention relates generally to HTTP streaming. More specifically, the invention relates to encapsulating a media entity for coded multi-component video streams such as scalable video coding (SVC) steams and multi-view coding (MVC) streams for HTTP streaming. 
       BACKGROUND OF THE INVENTION 
       [0004]    For HTTP streaming applications, at the server side, an encoded video is often encapsulated and stored as a file that is compliant with BMFF, such as an MP4 file. Moreover, to realize adaptive HTTP streaming, the file is usually divided into multiple movie fragments and these fragments are further grouped into segments, which are addressable by client URL requests. In practice, different encoded representations of the video content are stored in these segments, so that a client can dynamically choose the desired representation to download and playback during a session. 
         [0005]    Encoded layered video, such as an SVC or MVC bitstream, provides natural support for such bitrate adaptation by enabling different operating points, i.e., representations, in terms of temporal/spatial resolutions, quality, views, etc., by decoding different subsets of the bitstream. However, existing ISO Base Media File Format (BMFF) standards, such as the MP4 file format, do not support separate access of each layer or representation, and thus are not applicable to the HTTP streaming application. As shown in  FIG. 1 , in MP4 file format, the metadata for all the layers or representations for one media file are stored in the moov Movie Box, while the media content data for all the layers or representations are stored in the mdat Movie Box. In HTTP streaming, when the client requests one layer, the whole file has to be sent since all the layers or representations are mixed together and the client does not know where to find the required layer or representation. 
         [0006]    As will be seen later, in adaptive HTTP streaming applications, it is desirable to be able to reference media data samples, such as network abstract layer (NAL) units, across movie fragment or component file boundaries. In SVC/MVC context, such a reference may be built by using mechanisms like “Extractor”. Extractor is an internal file data structure defined in the SVC/MVC Amendments to the AVC file format extension of BMFF:  Information Technology—coding of audio - visual objects—Part  15 : Advanced Video Coding  ( AVC )  file format, Amendment  2 : File format support for Scalable Video Coding,  2008 ( pages  15-17). Extractor is designed to enable extraction of NAL units from other tracks by reference, without copying. Here track is a timed sequence of related samples in an ISO base media file. For media data, a track corresponds to a sequence of images or sampled audio. The syntax of Extractor is shown below: 
         [0000]    
       
         
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 class aligned(8) Extractor ( ) { 
               
               
                   
                   NALUnitHeader( ); 
               
               
                   
                   unsigned int(8)  track_ref_index; 
               
               
                   
                   signed int(8) sample_offset; 
               
               
                   
                   unsigned int ((lengthSizeMinusOne + 1) * 8) 
               
               
                   
                     data_offset; 
               
               
                   
                   unsigned int ((lengthSizeMinusOne + 1) * 8) 
               
               
                   
                     data_length; 
               
               
                   
                 } 
               
               
                   
                   
               
             
          
         
       
     
         [0007]    The semantics of the Extractor data structure are: 
         [0008]    NALUnitHeader: The NAL unit structure as specified in ISO/IEC 14496-10 Annex G for NAL units of type 20:
       nal_unit type shall be set to the extractor NAL unit type (type 31).   forbidden_zero_bit, reserved_one_bit, and reserved_three — 2 bits shall be set as specified in ISO/IEC 14496-10 Annex G.   Other fields (nal_ref_idc, idr_flag, priority_id, no_inter_layer_pred_flag, dependency_id, quality_id, temporal_id, use_ref_base_pic_flag, discardable_flag, and output_flag) shall be set as specified in B.4 of  Information technology—Coding of audio - visual objects—part  15 : Advanced Video Coding  ( AVC )  file format, Amendment  2 : File format support for Scalable Video Coding, ISO/IEC  14496-15:2004 /Amd.  2:2008 (page 17).       
 
         [0012]    track_ref_index specifies the index of the track reference of type ‘scal’ to use to find the track from which to extract data. The sample in that track from which data is extracted is temporally aligned or nearest preceding in the media decoding timeline, i.e. using the time-to-sample table only, adjusted by an offset specified by sample_offset with the sample containing the Extractor. The first track reference has the index value 1; the value 0 is reserved. 
         [0013]    sample_offset gives the relative index of the sample in the linked track that shall be used as the source of information. Sample 0 (zero) is the sample with the same, or the closest preceding, decoding time compared to the decoding time of the sample containing the extractor; sample 1 (one) is the next sample, sample −1 (minus 1) is the previous sample, and so on. 
         [0014]    data_offset: The offset of the first byte within the reference sample to copy. If the extraction starts with the first byte of data in that sample, the offset takes the value 0. The offset shall reference the beginning of a NAL unit length field. 
         [0015]    data_length: The number of bytes to copy. If this field takes the value 0, then the entire single referenced NAL unit is copied (i.e. the length to copy is taken from the length field referenced by the data offset, augmented by the additional_bytes field in the case of Aggregators). 
         [0016]    Further details can be found in  Information technology—Coding of audio - visual objects—part  15 : Advanced Video Coding  ( AVC )  file format, Amendment  2 : File format support for Scalable Video Coding, ISO/IEC  14496-15:2004 /Amd.  2:2008. 
         [0017]    Currently extractors are only able to extract, by reference, the NAL units from other tracks, but within the same movie box/fragment. In other words, it is not possible to use extractors to extract NAL units from a different segment or file. This restriction limits the use of extractors in the above use case. 
         [0018]    Prior solutions to the problems mentioned above have not adequately been established in the art. It would be desirable to provide the ability to parse and encapsulate layers without sacrificing speed and transport efficiency. Such results have not heretofore been achieved in the art. 
       SUMMARY OF THE INVENTION 
       [0019]    This invention directs to methods and apparatuses for encapsulating component files from a media entity containing more than one layer and for reading a component file. 
         [0020]    According to an aspect of the present invention, there is provided a method for encapsulating and creating component files from a media entity containing more than one layer. The method extracts metadata and media data corresponding to the extracted metadata for each layer from the media entity, and identifies references to additional media data related to the extracted media data for each layer. The references are embedded into the extracted media data for each layer. The extracted media data and metadata are associated to enable the creation, for each layer, of a component file containing the extracted metadata and the extracted media data. 
         [0021]    According to another aspect of the present invention, there is provided a file encapsulator. The file encapsulator includes an extractor for extracting metadata and media data corresponding to the extracted metadata for each layer from the media entity; a reference identifier for identifying a reference to additional media data, from the media entity, related to said extracted media data for each layer; and a correlator embedding the references into the extracted media data for each layer and for associating the extracted media data with the extracted metadata to enable creation, for each layer, of a component file. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The above features of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
           [0023]      FIG. 1  shows an example MP4 file format. 
           [0024]      FIG. 2  shows one embodiment of the current invention to encapsulate a media entity. 
           [0025]      FIG. 3  shows the structure of an Encapsulator used to encapsulate or create component files from a media entity which contain multiple layers/representations, 
           [0026]      FIG. 4  shows an example of associating additional media data with component files based on dependency relationship. 
           [0027]      FIG. 5  shows an example to extract an NAL unit, by reference, from a movie box/fragment that is different from the one that the extractor is within. 
           [0028]      FIG. 6  shows the involved encapsulation operations for an SVC/MVC type video bitsteam into multiple movie fragments using the invented new extractor data structure. 
           [0029]      FIG. 7  shows the structure of a file reader used to read the component files. 
           [0030]      FIG. 8  shows the process of reading an SVC/MVC type video bitstream for a video decoder involving the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    In present invention, a media entity, such as a media file or a set of media files or a streaming media, is divided or encapsulated into multiple movie component files, which are addressable by client URL requests. Here, a component file is used in a broader sense that it represents a fragment, a segment, a file and other equivalent terms thereto. 
         [0032]    In one embodiment of the present invention, a media entity containing multiple representations or components is parsed to extract metadata and media data for each representation/component. Examples of the representation/component includes layers, such as layers with various temporal/spatial resolutions and quality in SVC, and views in MVC. In the following, layers are also used to refer to representations/components, and these terms are used interchangeably. The metadata describes, for example, what is contained in the media entity for each representation and how to use the media data contained therein. The media data contain media data samples required for serving the purpose of the media data, e.g. decoding of the content, or any necessary information on how to obtain the required data samples. The extracted metadata and media data for each representation or layer are associated/correlated and stored together for user access. The storing operation can be done physically on a hard drive or other storing media, or can be performed virtually through a relationship management mechanism so that the metadata and media data appear to be stored together when interfacing with other applications or modules when they indeed are actually located in different places on storing media.  FIG. 2  illustrates an example of this embodiment. In  FIG. 2 , a media entity contains three layers: base layer, enhancement layer  1  and enhancement layer  2 . The media entity is parsed to extract the metadata and media data for each of the three layers, and those data are stored separately as component files with the metadata and corresponding media data associated together. 
         [0033]      FIG. 3  shows the structure of a preferred encapsulator  300  used to encapsulate and create component files from a media entity which contain multiple layers, such as SVC encoded videos. The inputs media entity  310  is passed to a metadata extractor  320  and a media data extractor  340 . The metadata extractor  320  extracts the metadata  330  for each layer. The media data extractor  340  takes in the metadata  330  and extracts the corresponding media data  350 . Note that in a different embodiment, the metadata extractor  320  and the media data extractor  340  are implemented as one extractor. Both data, metadata  330  and media data  350 , are feed into a correlator  380  which associates these two types of data and creates the output component files  390 , one component file for each layer. 
         [0034]    A layered video, such as a video encoded by AVC extensions of SVC or MVC, contains multiple media components (scalable layers or views). Such an encoded bitstream can provide different operating points, i.e., representations or layers, in terms of temporal/spatial resolutions, quality, views, etc., by decoding different subsets of the bitstream. Furthermore, there exist coding dependencies among the layers of the bitstream, i.e., the decoding of a layer may depend on other layers. Therefore, to request one of such a bitstream&#39;s representations may require retrieving and decoding one or more components or media data from the encapsulated video file. To facilitate the extraction process for different representations, an encoded layered video is often encapsulated into an MP4 file in a way that each layer is stored separately in different segments or component files. In this case, it needs to be taken into account that certain media data samples, such as NAL units, of the bitstream are required by, or related to, multiple segments or component files, due to the decoding dependencies described above or other dependencies based on the application. 
         [0035]    In another embodiment of the present invention, additional media data required by a segment or a component file are extracted and associated with the segment or component file.  FIG. 4  shows an example of this embodiment. In the figure, an SVC bitstream has three spatial layers, HD1080p, SD and QVGA. Three movie fragments or component files are formed corresponding to the three operating points, and each is addressable by a different URL. Inside each movie fragment or component file, all the media data samples, NAL units in this example, required for decoding are copied and stored as media samples contained in the “mdat” box. So, when a client requests a particular operating point or representation by using a proper URL, the server can retrieve the corresponding movie fragment or component file and forwarded to the client. In this embodiment, the media data extractor  340  in  FIG. 3  further extracts, for each layer, from the input media entity  310  additional media data related to the extracted media data for each of the layers. Correlator  380  further associates the additional extracted media data for each layer to create corresponding component files. 
         [0036]    For the sake of storage space saving, it is desirable to be able to reference media data samples, such as NAL units, across movie fragment or component file boundaries, without actually duplicating the same data in each component file. However, ISO Base Media File Format (BMFF) and its extensions currently do not support this feature. To solve this problem, in a further embodiment of the present invention, a reference is identified and built for those additional media data that are related to or required by the media data of a movie fragment or a component file. The reference, rather than those additional media data, is associated with the component file along with the metadata and media data thereof. One can embed the references into the extracted media data for each layer, and then associate the extracted metadata and extracted media data for each layer for creating corresponding component files. 
         [0037]    In this embodiment, a Reference Identifier  360  is added to the structure of the Encapsulator  300 . The Reference Identifier  360  identifies, from input media entity  310 , references  370  to those additional media data that are related to extracted media data  350  for each layer. Then references  370  are associated, via correlator  380 , with extracted metadata  330  and extracted media data  350  for each layer, e.g. by embedding said references into said extracted media data, for creating corresponding component files  390 . 
         [0038]    As discussed earlier, in SVC/MVC context, such a reference may be built by using mechanisms like “Extractor”. Currently extractors are only able to extract, by reference, the NAL units from other tracks, but within the same movie box/fragment. In other words, it is not possible to use extractors to extract NAL units from a different segment or file. This restriction limits the use of extractors in other cases. Hereafter, an extension to the extractor data structure is disclosed, where the extension is aimed to support efficient encapsulation of SVC/MVC type layered video content into multiple component files as described before. 
         [0039]    The extension is added to provide the extractor data structure with the extra capability to reference to NAL units that reside in a different movie box/fragment or component file other than the one in which extractor resides. 
         [0040]    The extended Extractor is defined as the following: 
         [0041]    Syntax: 
         [0000]    
       
         
               
             
           
               
                   
               
             
             
               
                 aligned (8) class DataEntryUrlBox (bit (24) flags) 
               
               
                   extends FullBox (’url’, version = 0, flags) { 
               
               
                   string location; 
               
               
                 } 
               
               
                 aligned (8) class DataEntryUrnBox (bit (24) flags) 
               
               
                   extends FullBox (’urn’, version = 0, flags) { 
               
               
                   string name; 
               
               
                   string location; 
               
               
                 } 
               
               
                 class aligned (8) Extractor ( ) { 
               
               
                   NALUnitHeader ( ); 
               
               
                   DataEntryBox (entry_version, entry_flags) data_entry; // added 
               
               
                 extension 
               
               
                   unsigned int(8) track_ref_index; 
               
               
                   signed int(8)  sample_offset; 
               
               
                   unsigned int ((lengthSizeMinusOne + 1) * 8) 
               
               
                     data_offset; 
               
               
                   unsigned int ((lengthSizeMinusOne + 1) * 8) 
               
               
                     data_length; 
               
               
                 } 
               
               
                   
               
             
          
         
       
     
         [0042]    Semantics: 
         [0043]    data_entry is a Uniform Resource Locator (URL) or Uniform Resource Name (URN) entry. Name is a URN, and is required in a URN entry. Location is a URL, and is required in a URL entry and optional in a URN entry, where it gives a location to find the resource with the given name. Each is a null-terminated string using UTF-8 characters. If the self-contained flag is set, the URL form is used and no string is present; the box terminated with the entry-flags field. The URL type should be of a service that delivers a file. Relative URLs are permissible and are relative to the file containing the Movie Box/Fragment that contains the track that the Extractor belongs to. 
         [0044]    Other fields have the same semantics as the original Extractor described before. 
         [0045]    With the extended extractor, it is now possible to extract a NAL unit, by reference, from a movie box/fragment that is different from the one the extractor is within.  FIG. 5  shows such an example, with the same SVC bitstream as  FIG. 4  but using the new extended Extractor data structure. As can be seen from the figure, now the SD movie fragment can reference to the NAL units from the QVGA movie fragments Likewise, the HD1080p movie fragment can use the extractors to reference NAL units from both QVGA and SD movie fragments. Compared to  FIG. 4 , no NAL units are duplicated across these movie fragments, thus storage space is saved. 
         [0046]      FIG. 6  shows the involved encapsulation operations for an SVC/MVC type video bitsteam into multiple movie fragments or component files using the invented new extractor data structure. The process starts at step  601 . Each NAL unit is read in one by one in step  610 . If the end of the bitstream is reached in step  620 , the process stops at  690 ; otherwise, the process proceeds to the next step  630 . Decision step  630  determines if the current NAL unit depends on NAL units from other track for decoding. If the determination is that the current NAL unit does not depend on NAL units from other tracks for decoding, the control is then transferred to step  640 , wherein a sample is formed using the current NAL unit and is placed in the current track. If the determination from step  630  is that there is dependency between the current NAL unit and NAL units from other track, the process goes on to step  650 . Decision step  650  further determines if the track from which NAL units are required by the current NAL unit resides within the same movie fragment. If the determination is that the track resides in the same movie fragment, step  670  is employed to fill in an extended Extractor to reference to the NAL unit from that other track. If the determination is that the track resides in a different movie fragment, the URL or URN of such a movie fragment is identified in step  660  and the process proceeds to step  670  with the identified URL and URN to be filled in an extended Extractor. After such an extended Extractor is filled in, it is embedded into the current track in step  680 . Then, the process starts over with the next NAL unit in step  610 . 
         [0047]    To read a component file, a file reader  700  shown in  FIG. 7  is employed. A parser  710  first parses the component file to get metadata and media data, and a reference if available. If, according to the decoded reference, the media data are related to media data of other component files such as through decoding dependency, a retriever  720  retrieves the related media data from other component files as indicated in the reference. A processor  730  further processes the metadata and media data obtained from the component file as well as the additional media data if available. The parsing operation by the parser  710  includes various necessary operations to obtain the metadata, the media data that are ready for the processor  730 , and the reference ready for the retriever  720 . It will include further parsing the metadata and/or the media data when necessary. In one embodiment, the reference is embedded in the media data, and thus the reference is obtained by parsing the media data. If a reference is available, the parsing step further includes analyzing the syntax of the reference and decoding the reference. The processor  730  can contain a video decoder if the component file contains video content. In a different embodiment, the parser and the retriever can be incorporated in the processor. 
         [0048]      FIG. 8  shows the process of reading an SVC/MVC type video bitstream for a video decoder involving the present invention. Step  801  accesses a component video file whose metadata and media data for each layer are identified in step  805 . The identified metadata and media data are parsed in step  810  and each NAL unit of the media data is read in one by one in step  815 . For the current NAL unit, a decision is first made at step  820  to determine if the end of the bitstream is reached, and the process ends at step  825  if the answer is “Yes”. Otherwise, the process proceeds to decision step  830  to determine if the current NAL unit is an extractor. If it is not an extractor, which means it is a normal NAL unit containing decoding data, the NAL unit is sent to decoder at step  835 . If the current NAL unit is an extractor, it is determined at step  840  that whether the current NAL unit depends on a NAL unit outside the same component file or not. If the required NAL unit is within the same component file, it is retrieved from the current file in step  845  and sent to the decoder at step  835 . If the required NAL unit is from another component file, the NAL unit is located using the Data_entry information in the extractor in step  850 , retrieved from the remote file in step  855  and then sent to the decoder in step  835 . 
         [0049]    Although preferred embodiments of the present invention have been described in detail herein, it is to be understood that this invention is not limited to these embodiments, and that other modifications and variations may be effected by one skilled in the art without departing from the scope of the invention as defined by the appended claims.