Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
  • H04N21/20—Servers specifically adapted for the distribution of content, e.g. VOD servers; Operations thereof
  • H04N21/23—Processing of content or additional data; Elementary server operations; Server middleware
  • H04N21/235—Processing of additional data, e.g. scrambling of additional data or processing content descriptors
  • H04N21/2353—Processing of additional data, e.g. scrambling of additional data or processing content descriptors specifically adapted to content descriptors, e.g. coding, compressing or processing of metadata
• • G—PHYSICS
  • G06—COMPUTING OR CALCULATING; COUNTING
  • G06T—IMAGE DATA PROCESSING OR GENERATION, IN GENERAL
  • G06T9/00—Image coding
  • G06T9/40—Tree coding, e.g. quadtree, octree
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03M—CODING; DECODING; CODE CONVERSION IN GENERAL
  • H03M7/00—Conversion of a code where information is represented by a given sequence or number of digits to a code where the same, similar or subset of information is represented by a different sequence or number of digits
  • H03M7/30—Compression; Expansion; Suppression of unnecessary data, e.g. redundancy reduction
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
  • H04N21/20—Servers specifically adapted for the distribution of content, e.g. VOD servers; Operations thereof
  • H04N21/23—Processing of content or additional data; Elementary server operations; Server middleware
  • H04N21/235—Processing of additional data, e.g. scrambling of additional data or processing content descriptors
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N21/00—Selective content distribution, e.g. interactive television or video on demand [VOD]
  • H04N21/40—Client devices specifically adapted for the reception of or interaction with content, e.g. set-top-box [STB]; Operations thereof
  • H04N21/43—Processing of content or additional data, e.g. demultiplexing additional data from a digital video stream; Elementary client operations, e.g. monitoring of home network or synchronising decoder's clock; Client middleware
  • H04N21/435—Processing of additional data, e.g. decrypting of additional data, reconstructing software from modules extracted from the transport stream

Definitions

• the field of the invention is that of the compression of data. More precisely, the invention regards the compression of XML-based document ("extended Markup Language ”) .
• the invention has applications, in particular, but not only, in the following fields : - multimedia applications ; indexation tools ; meta-data manipulation tools ; - the MPEG-7 specification ;
• Compression techniques of the prior art for XML have several drawbacks. In particular, they do not support at the same time fast access to data, high compression ratios and progressive construction of the document. In other words, most of the time, when one of the above mentioned feature is supported, all other features are missing.
• BiM Binary MPEG
• Such a technique provides a method for compressing a XML document by binarising the structure of the document, that it to say the nodes of a tree structure associated to the XML document.
• the compression ratio achieved by implementing the BiM technique is very poor, although the BiM technique allows a fast access to data, progressive construction of the document and skippability.
• the invention aims at providing an efficient compression technique for XML-based documents.
• the invention also aims at providing a compression technique for XML which provides skippability, high compression ratios and progressive construction of the document.
• the invention also aims at compressing efficiently MPEG-7 descriptors.
• Another aim of the invention is to implement a method for compressing an MPEG-7 descriptors.
• aims of the invention are achieved, according to the invention, by means of a method for compressing a hierarchical tree describing a multimedia signal, said tree comprising nodes and leaves, which can be associated to contents of at least two distinct types, wherein said method implements a content compression for at least some of said leaves by means of at least two compression encoding techniques, each of said techniques being selectively associated to at least one of said content types.
• such a method comprises a step of identifying at least one sub-tree and a step of allocating one of said compression encoding techniques to said sub-tree.
• such a method comprises a step of implementing said compression encoding technique allocated to said sub-tree only for the leaves of said sub-tree whose content is of the type associated to said compression encoding technique, and the other leaves of said sub-tree do not undergo any compression encoding.
• such a method implements a parametrical description of said compression encoding techniques.
• such a method also comprises a step of compressing the structure of said tree.
• said tree is of the BiM (Binary MPEG) type according to the MPEG7 standard.
• Preferentially one of said compression encoding techniques implements linear quantization.
• one of said compression encoding techniques implements a statistical compression algorithm.
• said algorithm is of the GZip type.
• said algorithm is simultaneously implemented for a set of data corresponding to the content of at least two leaves.
• said tree represents the structure of an XML (Extended markup language) type document.
• the invention also regards a method for decoding a multimedia signal compressed according to the above-mentioned method for compressing a hierarchical tree.
• such a method implements a step of refreshing a present decoding context according to encoding context information conveyed by said signal.
• said present context defines at least one content type, said method comprising a step of implementing a compression decoding technique associated to said content type for the leaves having a content of said content type.
• the invention also regards a signal generated by the above-mentioned method for compressing a hierarchical tree.
• figure 1 illustrates the concept of coding context
• - figure 2 describes the structure of an element as coded according to the BiM technique
• figure 3 illustrates some of the steps implemented according to the invention for compressing the content of the leaves of a hierarchical tree.
• a coding context illustrated in figure 1, is a set of decoding information, needed while decoding the bitstream.
• a coding context is applicable to the whole sub-tree of the node where it is defined. At every nodes of the tree, the coding context can be modified ; leading to the creation of a new coding context, applicable to the corresponding sub-tree.
• a context can carry several information which edict features applicable to the concerned sub-tree.
• these features are skippability of a sub-tree/context and multiple schema encoding of a sub-tree/context (in order to provide the backward and forward compatibility feature) .
• the context mechanism can be disabled in every sub-tree in order to save bandwidth ; this is the context frozen mode.
• the coding context mechanism provides maximum flexibility in every sub-tree of a document tree and allows extensible features to be plugged into the BiM encoding mechanism.
• a codingContext is a set of information, the contextual information, needed by the decoder to decode the bitstream.
• a codingContext is applicable to the node where it has been defined, and the whole sub-tree corresponding to this node.
• the current codingContext (i.e. the context applicable at a specified node of a description) can be modified within the document (that is to say, a modification of its underlying set of information).
• Each modification of a codingContext leads to the creation a of new codingContext, which will carry the modified set of information.
• All codingContexts are expected to be stacked, in order to get them back, when the decoder has finished decoding a sub-tree corresponding context.
• the BiM decoder is composed of two decoders :
• this decoder is dedicated to decode the contextual information.
• the contextual information is not part of the description. This is a set of information which carry some external features, backward and forward compatibility, fast skipping...
• the BiM regular one [1] is dedicated to decode the element information.
• the MC metacontext chunk contains the information needed by the decoder to decode the following C chunk. That is to say that the MC chunk is the context chunk of the C context chunk.
• the C context chunk contains the information able to change the current coding context set of information, and needed by the decoder to decode the following element chunk. That is to say that the C chunk is the context chunk of the element chunk.
• the current BiM coding context carries a set of information, the contextual information, which can be divided in the two following main classes :
• the current set of information is the following set of variables :
• the MC metacontext chunk which size can be null, contains information to know if the decoder has to read the next C context chunk, described in the following section.
• the default value of freezing_state is false ; that is to say that, by default, the root context can be dynamically changed.
• the freezing_state value is set to the freezing_state value of its father's context
• the MC metacontext chunk (and the upcoming C context chunk) is not coded into the bitstream. Otherwise, the MC metacontext chunk part of the header is coded as follows:
• the context_chunk is a : ocal variable, initialised at false. freeze type Implication
• the decoder has to read the following C context chunk.
• the C context chunk which size can be null, contains a set of information able to dynamically change the current context variables. These variables are called codingProperties because they influence the BiM element decoding process.
• the allows_skip variable is initialized at the beginning of the bitstream by the first two bits of the special 4 bits bitfield, as defined in the FCD Systems document [1].
• the allows_partial_instantiation variable is initialized at the beginning of the bitstream by the third two bits of the special 4 bits bitfield.
• the allows_subtyping variable is initialized at the beginning of the bitstream by the fourth two bits of the special 4 bits bitfield.
• schema_mode The default value of schema_mode is mono ; that is to say that, by default, the root sub-tree/context is encoded with one schema.
• - schema_mode can be dynamically modified Decoding rules
• the C Context chunk is present only if the MC metacontext chunk is already present and its previous local variable context_chunk is true.
• the dynamic modification of the current context is described with an XML element which is encoded with the BiM regular encoding scheme.
• the global element modifyContext from the BiM schema is used.
• the htt //www .mpeg7.org/2001 BiMCoding coding schema is described in annex 1.
• the C context chunk has to be decoded with the BiM regular scheme, with the above schema.
• the modification of the current codingProperties in the context implies the creation of a new context. Therefore, the presence of the C context chunk, implies the creation of a new context, which will carry the modified codingProperties.
• allowsSkip element is instantiated within the modifyContext element, then the value of allows_skip will be updated in the new context.
• schema_mode element If the schema_mode element is instantiated within the modifyContext element, then the value of schema_mode will be updated in the new context. Influence on the element decoding process
• the allows_skip and schema_mode values influence the element decoding process, when dealing with the skipping feature. This behavior is described in [1] .
• the schema_mode value influences the element decoding process, in order to know if the element is coded with only one schema or several ones. This mechanism is described in [1].
• the allows_partial_instantiation value influences the element decoding process, by adding one special type partially Instantiated type to the possible subtypes of the element. See [1].
• the allows_subtyping value influences the element decoding process, and allows an element or an attribute to have different possible types, in case of element polymorphism (with the xsi:type attribute) or union. See [1]. 2. Description of the invention
• the invention proposes to extend the current BiM context mechanism in order to support a new and interesting feature : the use of local compressors to compress leaves of a document, in order to reduce the size of the resulting bitstream.
• This section describes how to extend the current BiM context mechanism to support the use of local compressors. This is typically a new set of variables, codingProperties, linked with specific semantic, propagation and coding rules. Therefore, this new set of codingProperties will extend the current context chunk.
• all the instances of one or several specified simple type can be compressed with one or several specified compressor. This basically defines a mapping between a compressor and one or several simple types. Moreover : - in some cases, a compressor can need some external parameters
• mapping can be activated/deactivated, in order to use a compressor in some sub-trees but not in another ones
• each context can carry zero, one or several codecTypeMapper ; where a codecTypeMapper is a 4-plet, consisting of an identifier, one or several simple types, a codec, optional external codec parameters and an activation state. Definitions CodecTypeMapper A codecTypeMapper is a 4-plet, consisting of :
• mapping is applicable a codec optional external codec parameters (depends of the codec) - an activation state
• the identifier is an unique number which identify a mapping within a context in an unambiguous way.
• the BiM coding schema restricts the maximal number of codecTypeMappers in a context to 32. Simple type
• a codec standing for compressor/decompressor, is a module which takes input bits, and writes output bits. It can need some optional external parameters.
• a codec is identified by a name, among the names of non-abstract codecs defined in the BiM coding schema.
• the current BiM coding schema defined in a section above, doesn't define any non-abstracts codecs, but ⁇ 2.2 of the present document does.
• the activation state is a boolean flag.
• codecTypeMappers - can carry zero, one or several codecTypeMappers - can define one or several codecTypeMappers
• codecTypeMapper If a codecTypeMapper is defined in a context, it remains in all its subcontexts. An existing codecTypeMapper, within a context, cannot be deleted nor modified (except its activation state). Identifier
• the identifier of a mapping must be unique among all the codecTypeMappers of a context.
• codecs There are two types of codecs : memoryless codecs and contextual codecs.
• a memoryless codec is a module which encodes always the same input bytes into the same bytes out ; independently of the history of the codec.
• a typical memoryless codec is a linear quantifier.
• the BiM leaf compression (see ⁇ 2.2 of the present document) describes such a codec.
• a contextual codec is a module which uses the previous bytes fed in it, (thus changing the context of the codec). Such a codec doesn't generate the same output bytes for the same input bytes it receives.
• a typically contextual codec is a Ziplike local codec, one is described in ⁇ 2.2 of the present document.
• a memoryless codec doesn't induce any problem in the current context architecture but a contextual codec does, in case of skippable sub-tree. In such cases, a contextual codec is reset, in order not to confuse the decoder, when this former has skipped the sub-tree.
• a codecTypeMapper can be activated or deactivated.
• This new codingProperty is named codecTypeMapper and is a list of the previous codecTypeMapper described in the previous section.
• codecTypeMapper By default, there is no codecTypeMapper in a sub-tree/context. If a codecTypeMapper is defined within a context, its identifier, codec and simple_ty ⁇ e value must be defined. If not specified, the state of activation of a newly defined codecTypeMapper is set to true by default ; that is to say that a newly defined codecTypeMapper is activated by default. New propagation rules
• the decoder is expected to create a new instance of the codec by copying the instance of the father's codec (not only its value), and resetting it.
• a ZLib codec would be copied and re-initialized when entering a skippable node.
• the example, illustrated in annex 3, presents the definition of one activated linear quantifier (see ⁇ 2.2 of the present document) in a description.
• the example, illustrated in annex 4 presents the definition of one deactivated linear quantifier in a description.
• Such a mechanism is closely related to the coding context and allows the use of several other types of codecs. Moreover, it allows to deal properly with coding context features, for instance skippable subtrees. Finally it allows re-use of codecs in different coding contexts.
• the BiM sub-tree coding [1] doesn't compress the data leaves of a description.
• leaf values are encoded with respect of their types (IEEE 754 floats and doubles , UTF strings ... ) .
• Linear quantization is an usual and lossy way to reduce the size of encoded numbers in the bitstream, when the source of the information is known and therefore, when losses can be controlled.
• the envelope of a sampled audio signal is often known with a precise bitsize quantization, and this technique could be fruitfully used for coding MPEG-7 audio descriptions.
• v a real number
• v q nbits bits
• - v min is the minimal inclusive value that v can reach - v max is the maximal inclusive value that v can reach
• v is the decoded, approximated value of v Integration with the context mechanism : the LinearQuantizerCodec
• Linear quantization can be used as a codec, as defined in the coding context mechanism described in ⁇ 2.1 of the present document. With this mechanism, linear quantization can be applied on numerical data leaves, of a desired simple type, in any sub-tree of a description. Used like this, the coding context mechanism, associated with the linear quantization codec, is acting as the QuantizationParameter node, used in MPEG-4 BIFS [3]. Restriction on applicable simple types
• this codec is a memoryless codec, which can be applied on every atomic and non- atomic simple numerical types ; whose XML Schema primitive type is float, double or decimal.
• the linear quantizer codec needs the following 3 mandatory parameters :
• a numerical data leaf of value v is encoded with the unsigned integer v q on nbits bits where :
• Vq V-Vmir (2'"" -l) Vmax —Vmin Decoding
• Example (informative) The example illustrated in annex 6 presents the definition of a linear quantizer in a description. 22.2. Statistical compression
• This codec is useful for significantly reducing the size of the bitstream, especially when the description contains many repetitive or similar strings.
• a buffered statistical coder relies on an underlying statistical coder which should contain the generic following primitives methods :
• a buffered codec has a bufferSize bytes length, byte array buffer FIFO structure. From the encoder side, the buffers ize value indicates how many input bytes the encoder can process before flushing. From the decoder side, this is the minimal buffer size in bytes, needed to decode the bitstream, through the underlying statistical coder API.
• the buffer has also a fillingLevel variable, which contains the actual filling level, in bytes, of the buffer.
• the ZLib public library API [4] used in the GZip compression scheme, provides an efficient and useful API for using statistical compression on document leaves.
• the ZLib API fulfils the previous generic methods, with the following mapping :
• - reset_model() can be mapped with an ZLib's inflateEnd() or a deflateEnd() call and a following i ⁇ tialize_stream() Call.
• - feed_input_bytes() can be mapped with the ZLib's deflate() method with the Z_NO_FLUSH parameter.
• - flush_output_bytes ⁇ can be mapped with the ZLib's deflate() method with the
• this codec is a contextual codec, which can be applied on every atomic and non-atomic string types.
• the ZLibCodec is relying on the underlying primitive encoding of leaves of a document, as described in [1]. For instance, int leaves are encoded with a 32 bits unsigned integer, string with a UTF-8 encoding, float and double are encoded with the IEEE 754 format, ... Therefore, the ZLibCodec will compress the encoded leaf
• the buffered ZLib codec doesn't need any external parameters, as the efficiency of the underlying ZLib is set at Z_DEFAULT_COMPRESSION and as the bufferSize parameter is not needed from the decoder side.
• the ZLib codec is a new codec of type ZLibCodecType, based on the abstract CodecType (see ⁇ 2.1) type and defined by the schema illustrated in annex 7, in the coding context namespace.
• Encoding (informative) At the activation/instantiation of the codec : the FIFO buffer structure is supposed to be clear, its fillingLevel is set to 0 - the global variable referencable_chunk is initialized to null
• the referencable_chunk should contain a referencable chunk of bits, which must be hold by the encoder, because its value will be known later during the encoding process.
• the signaling function signal_reference_chunk_known() could be called when this chunk is known.
• the size, in bytes, of every non-nil chunk should be write before the chunk itself, during the flush_output_bytes0 call, with the standard unsigned infinite integer 4+1 coding, as defined in [1].
• An input leaf is the encoded value of a textual leaf, with respect of its primitive type.
• the length of the leaf, in bytes, is given by the field leaflength.
• a string leaf is an UTF-8 code, preceded by the size in bytes of the string (coded with the infinite integer coding [1]) ;
• a double leaf is the 64-bits value of the corresponding IEEE 754 standard...
• Decoding is defined by : 1. If the FIFO is empty : a. decode the coded data, b. stack in a FIFO all elements separated by 0x00 c . if the last character is not 0x00 store the unfinished string temporarily. d. if "last_element" is not empty, insert it at the beginning of the first element in FIFO e. put the unfinished string of this round in last_element. f . remove and return the first element. 2. If the FIFO is not empty, then remove and return the first element. It is equivalent to say : "the FIFO is not empty” and to say "there is no encoded data in a the current leaf.”
• annex 8 The description given in annex 8 is an example of the usage of the ZLibCodecType codec, mapped with the string and the anyURI types.
• the following figures show the performances of using the ZLibCodec so as to compress textual leaves of descriptions (those derived from the string and the anyURI XML Schema primitive types).
• a buffer of bufferSize 256 bytes was used during the encoding process.
• the files used were provided by the MPEG-7 MDS sub-group.
• Step 1 consists in associating a compression encoding technique to a content type.
• linear quantization can be associated to floating point values.
• Step 2 a sub-tree is identified within the hierarchical tree corresponding to the structure of the considered XML document.
• Step 3 consists in allocating a compression encoding technique, to the identified sub-tree.
• Step 4 then consists in checking whether the codec implementing the compression encoding technique is or not activated. If no, no compression (5) of the leaves of the sub-tree is achieved.
• the invention implements (6) compression of the content of the subtree leaves whose content is of the content type associated (1) to the compression encoding technique.

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