Abstract:
Document description files are used to encapsulate structural and meta information associated with a document stored on a computer-readable medium. Document description files are external to native application files and have a set of required fields. Document description files point to the referenced document data using Uniform Resource Locators (URLs) and serve as virtual documents. In addition to the required fields, applications can choose to encode additional structural information in the document description files.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to computer file formats capable of enhancing cross-application interoperability. 
     Computer content authoring application programs, such as word processing applications and spreadsheet applications, produce content having varying levels of structure. Furthermore, different authoring applications store content and structural information using different file formats. Typically, the file format used by one authoring application is not understood by other authoring applications. As a result, the content and structural information stored in a file is typically accessible only to the authoring application that produced the file and to other authoring applications which have been specifically designed to understand the file format of the file. Authoring applications which have not been specifically designed to understand the file format of the file may be able to retrieve the content stored in the file, but typically will not be able to retrieve the structural information stored in the file. 
     In networked environments where there is a large amount of communication among applications and among computers, requiring each application to understand the file formats used by other applications is becoming increasingly unwieldy and inefficient. One method which has been used to address this problem is the use of a “universal” file format which attempts to encapsulate all possible content and structural information that can be generated by any application program. Files stored in such a format, however, tend to be large, and cannot be guaranteed to be capable of encapsulating information generated by future versions of application programs. 
     SUMMARY OF THE INVENTION 
     In one aspect, the invention features a method executed on a computer for generating a first document description file for describing a document stored on a computer-readable medium. The method generates a description of an application which produced the document, generates a description of a location from which the document can be obtained, and generates a description of an operation that can be performed on the document to produce a second document description file. The description of the location may be a uniform resource locator. The uniform resource locator may identify a server configured to produce the document upon request. The uniform resource locator may identify a location at which the document is stored. The content of the first document description file and the content of the second document description file may be represented in XML syntax. 
     The operation may be a transformation of the document from a file stored in a first storage format to a file stored in a second storage format, and the second document description file describes the file stored in the second storage format. The second document may describe the first document description file. The operation may extract information from the document, and the second document description file describes the information extracted from the document. The second document description file may describe the first document description file. The information extracted from the document may describe a range of pages of the document. The document may represent a multi-layered graphical object, and the information extracted from the document describes a subset of the layers of the multi-layered graphical object. 
     The method may generate application-specific data describing the document. The application-specific data may be a name of an application that produced the document. The application-specific data may be a version number of an application that produced the document. 
     The method may generate a field containing information describing the document. The field may be an HTTP header. The field may describe a date on which the document was produced. The field may describe a date on which the document was modified. The field may describe a size of the document. The field may describe content contained in the document. 
     In another aspect, the invention features a method for processing a request document description file stored on a computer-readable medium, the request document description file describing a source document and an operation to be performed on the source document. The request document description file is received from a client, the source document is retrieved, the operation is applied to the source document to produce information derived from the source document, and a response document description file is generated containing a description of the information derived from the source document. 
     The information derived from the source document may be a result document. The response document description file may be a pointer to the result document. The description of the information derived from the source document may be the result document. The description of the information derived from the source document may be a pointer to the result document. The pointer may be a uniform resource locator. The response document description file may be generated by generating a description of the source document. The description of the source document may be the source document. The description of the source document may be a pointer to the source document. The pointer may be a uniform resource locator. The response document may be transmitted to the client. The information derived from the source document may be transmitted to the client. 
     In another aspect, the invention features a document description file, stored on a computer-readable medium, for describing a document stored on a computer-readable medium, the document description file containing a description of an application program which produced the document, a description of a location from which the document can be obtained, and a description of an operation that can be performed on the document to produce a second document description file. The operation that can be performed on the document may be a transformation of the document from a file stored in a first storage format to a file stored in a second storage format, and the second document description file describes the file stored in the second storage format. The operation that can be performed on the document may be extraction of information from the document, and the second document description file describes the information extracted from the document. The document description file may contain a description of an operation to be performed on the document. 
     A document description format (DDF) file encapsulates the location of a document along with useful descriptive information about the document. This enables authoring applications to capture and export information about content without requiring changes to current authoring application file formats. A DDF can be used as a virtual document to capture as much or as little information about data contained in a native authoring application file as is desired by the application and/or user. 
     Among the advantages of the invention are one or more of the following. 
     One advantage of the invention is that the content of a document description format (DDF) file is independent of the authoring application used to produce the file described by the DDF. Different authoring applications can therefore use DDF files to cooperatively manipulate, synthesize, and exchange document data. Although DDFs are independent of application-specific data, application-specific data can optionally be encapsulated within a DDF in order to optimize certain operations. 
     Another advantage of the invention is that the size of a DDF file is typically much smaller than the document which it describes. A typical DDF file is a few hundred bytes long. This aspect of the invention is particularly advantageous when used in conjunction with files, such as multimedia files, which are typically very large. Because the size of a DDF file is independent of the size of the file described by the DDF, the size of a DDF file will typically not increase if the size of the document described by the DDF increases. Local storage of DDF files instead of native files can therefore result in significant storage savings. 
     Another advantage of the invention is that, as a result of the small size of DDF files, exchange of DDF files is more efficient than exchange of the native files described by the DDFs. Many user-level manipulation tasks involve retrieving a diverse set of data items, assembling them, filtering out certain data items, and eventually creating a data aggregation consisting of the data items of interest. The intermediate steps of such operations can be performed more efficiently by using DDFs than by using native files, because of the small size of DDF files and the selective encapsulation of structural information and meta-data provided by DDFs. 
     Another advantage of the invention is that DDF-aware clients, servers, and applications use late binding, i.e., a reference within a DDF to a native file is not bound to the content of the native file until it is actually necessary to access such content, such as when the file is to be printed. Use of late binding reduces the number of temporary files that are produced when performing a series of operations on a file, thereby increasing the efficiency of such operations. 
     Another advantage of the invention is that implementing use of the document description format requires minimal modifications to existing application programs. A simple plug-in to an existing application program can be used to enable the application program to save descriptions of existing documents. DDF client software running on a client workstation manipulates DDF files and handles client-server DDF transactions without requiring any modification to existing application programs. 
     Another advantage of the invention is that it makes more efficient use of client resources. For many transactions, clients need only store and exchange DDF files which are typically much smaller than the application-specific files to which they correspond. Furthermore, because all processing of application-specific files is performed by application-specific servers, the number of authoring applications that clients need to store and execute is reduced. Furthermore, application-specific servers can be optimized to process files produced by specific applications, thereby increasing processing efficiency. 
     Other features and advantages of the invention will become apparent from the following description and from the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a network and computer hardware and software configured to manipulate document description format files. 
     FIG. 2 is a block diagram of the communications that take place among a subset of the elements shown in FIG.  1 . 
     FIG. 3 is a flowchart of a method for applying a transformation to a document description format file. 
     FIG. 4 illustrates a computer and computer elements suitable for implementing the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, an authoring application APP 1   12 , such as a word processing application or a graphics application, resides on a client workstation  10 . A user uses the authoring application  12  to produce a document having raw information content, and to save the content on a local disk  16  in a file  18  formatted in a native file format of the authoring application  12 . If the user desires to access the native APP 1  file  18 , the user instructs the authoring application  12  to load the native file  18  from the disk  16  into the memory of the client workstation  10 . The user may then manipulate the file  18  in the memory of the client workstation  10  using the authoring application  12 . 
     Also residing on the client workstation  10  is a document description format (DDF) plugin  14  to the authoring application  12 . A DDF file, described in more detail below, provides an application-independent description of a document saved in a native file format of an authoring application program. The DDF plugin  14  provides the authoring application  12  with the ability to save and retrieve DDF files without requiring any modifications to the authoring application  12 . Alternatively, the DDF plugin  14  may save information to and retrieve information from transitory data structures in the memory of the client  10 , rather than using DDF files. A separate application or script (not shown) provides a batch operation to produce DDF files corresponding to pre-existing native files. 
     The DDF plugin  14  interacts with the user by, for example, adding a command to the authoring application  12  which allows the user to save a DDF file corresponding to the file currently open in the authoring application  12 . In the case of an authoring application which uses a graphical user interface (GUI), the added command may take the form of a “Save DDF file” menu item in the authoring application&#39;s “File” menu. A user&#39;s selection of this menu item from within the authoring application  12  causes the DDF plugin  14  to produce a DDF file  20  describing the native file  18 , and to optionally prompt the user for additional meta information (e.g., the author&#39;s name). The user could also be provided with an option to have the authoring application&#39;s default “save file” operation always generate an accompanying DDF file. Alternatively, instead of using the DDF plugin  14 , the authoring application  12  may be modified to allow the user to save and retrieve DDF files. 
     As previously noted, a DDF file describes a document produced by an authoring application. The document described by a DDF file is referred to as the “referenced document” of the DDF file. A DDF file contains three kinds of information about the DDF file&#39;s referenced document: (1) meta information such as the location and authoring application of the referenced document; (2) method declarations enumerating the operations that can be performed on the referenced document; and (3) optional application-specific data describing the referenced document. 
     Referring to FIG. 2, the authoring application APP 1   12  saves content in native file  18 . The authoring application APP 1   12  uses DDF plugin  14  to save a DDF file  20  describing the native file  18 . A user of authoring application APP 2   32 , residing on client workstation  30 , desires to obtain a copy of native file  18  transformed into Graphics Interchange Format (GIF). The user instructs DDF client software  34 , residing on client workstation  30 , to construct a request DDF file  42 , containing a request to transform native file  18  into GIF format, from the information contained in APP 1  DDF file  20 . DDF client software  34  transmits the request DDF file  42  to APP 1  DDF servlet software  26 , residing on an APP 1  server  24 . Alternatively, DDF client software  34  transmits the APP 1  DDF file  20  and a separate request (not shown) to APP 1  DDF servlet software  26 . 
     APP 1  DDF servlet software  26  transforms the native APP 1  file  18  into GIF format and stores the resulting GIF file  22  on disk  16 . APP 1  DDF servlet software  26  constructs a response DDF file  44 , which encapsulates the request DDF file  42  and describes the GIF file  22 , and transmits the response DDF file  44  to the DDF client software  34 . The DDF client software transmits information contained within the response DDF file  44 , such as the location of the GIF file  22 , to the authoring application APP 2   32 . Authoring application APP 2   32  uses standard web browser software  36 , residing on client workstation  30 , to retrieve the GIF file  22 . 
     In one implementation, a DDF file minimally contains: (1) header fields, such as Hypertext Transfer Protocol (HTTP) 1.1 header fields, describing properties of the referenced document, such as its title and the date on which it was produced; (2) a field specifying the authoring application that produced the referenced document; and (3) an address or location of the referenced file, such as a Uniform Resource Locator (URL). An example of a DDF file that references an Adobe Photoshop document is shown in Table 1. 
     
       
         
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 &lt;DDF&gt; 
               
               
                   
                  &lt;URL VALUE=“http://www.company.com/doc.psi”/&gt; 
               
               
                   
                  &lt;Date VALUE=“Mon Aug 4 09:48:55 PDT 1997 ”/&gt; 
               
               
                   
                  &lt;Title VALUE=“Picture of House”/&gt; 
               
               
                   
                  &lt;Content-type VALUE=“Application/vnd.adobe-photoshop”/&gt; 
               
               
                   
                  &lt;Methods&gt; 
               
               
                   
                   &lt;Transformation NAME=“PhotoshopToGif” 
               
               
                   
                    RETURN-TYPE = “Image/Gif” 
               
               
                   
                    PROVIDER = “http://ddf.company.com/ptg.class”/&gt; 
               
               
                   
                   /&gt; 
               
               
                   
                   &lt;Information NAME=“EnumLayers” 
               
               
                   
                    RETURN-TYPE = “Layers” 
               
               
                   
                    PROVIDER = “http://ddf.company.com/el.class”/&gt; 
               
               
                   
                   /&gt; 
               
               
                   
                  &lt;/Methods&gt; 
               
               
                   
                  &lt;Application-data&gt; 
               
               
                   
                   &lt;Application-name VALUE = “Adobe Photoshop”/&gt; 
               
               
                   
                   &lt;Application-version VALUE = “4.0”/&gt; 
               
               
                   
                  &lt;/Application-data&gt; 
               
               
                   
                 &lt;/DDF &gt; 
               
               
                   
                   
               
             
          
         
       
     
     As shown in Table 1, all DDF elements (e.g., “DDF”, “Methods”) are encoded in Extensible Markup Language (XML) syntax, and can therefore be parsed by a conforming XML parser even if that parser does not understand the semantics of application-specific data contained in the DDF file. DDF element and attribute names are case insensitive. 
     In the DDF file shown in Table 1, the element labeled DDF indicates that the file is a DDF file. The VALUE attribute of the URL element specifies the location of the DDF file&#39;s referenced document. The VALUE attribute of the Content-type element indicates the Multipurpose Internet Mail Extensions (MIME) content type of the referenced document. 
     The optional Application-data element contains information about the referenced document that is specific to the authoring application that produced the referenced document. For example, Application-data might include information about the individual layers of a multi-layered object produced by a graphics application, or information about the location of tab stops in a word processing document. Any information contained within the Application-data element need only be capable of being understood by components that directly manipulate the referenced document (e.g., the application APP 1  DDF servlet  26  shown in FIGS.  1  and  2 ). Specifically, DDF client software  34  need not understand the information contained within the Application-data element in order to properly process and manipulate DDF files. Note that DDF files which do not contain an Application-data element still contain sufficient information to enable the retrieval of all application-specific information relating to a referenced document by making a request to an appropriate application-specific server. 
     The optional Methods element declares methods that can be performed on a DDF file&#39;s referenced document to produce information derived from the referenced document. A method may be either a transformation or an information method, as indicated by the Transformation and Information elements, respectively. Transformation methods, when applied to a source DDF, return a response DDF file that describes a transformation of the source DDF&#39;s referenced document. Alternatively, a transformation method may return the actual result of applying the transformation method to the referenced document. For example, the transformation method declared by the Transformation element in the DDF file shown in Table 1 transforms the DDF file&#39;s referenced document (an Adobe Photoshop file) into a Graphics Interchange Format (GIF) file, and returns the resulting GIF file. Information methods, when applied to a source DDF, return a response DDF file that contains additional information about the source DDF&#39;s referenced document. For example, application of the Information method declared in the DDF file shown in Table 1 to a source DDF produces a response DDF file containing a Layers element which contains information about the layers in the source DDF&#39;s referenced document (an Adobe Photoshop file). 
     Both Transformation and Information elements may contain a NAME attribute (describing a name of the method declared by the element) and a PROVIDER attribute (providing a pointer to an implementation of the method declared by the element). Both Transformation and Information elements may have a RETURN-TYPE attribute. For Transformation elements, the RETURN-TYPE attribute specifies the MIME type of the document returned when the method declared by the element is applied. For example, in the DDF file shown in Table 1, the RETURN-TYPE of the PhotoshopToGif transformation method is “Image/Gif,” indicating that the result of applying the PhotoshopToGif method is a GIF image. For Information elements, the RETURN-TYPE attribute specifies the name of the DDF element whose content is returned as a result of applying the Information method. For example, the RETURN-TYPE of the Information element in the DDF file shown in Table 1 is “Layers.” This indicates if a request DDF file, requesting execution of the “EnumLayers” Information method, is transmitted to an appropriate Photoshop server, the resulting response DDF file will contain a Layers element containing information about the layers of the Photoshop document referenced by the DDF file shown in Table 1. Attributes common to all methods within a Methods element are optionally listed once as attributes of the enclosing Methods element. 
     Message-IDs may be used to uniquely identify DDF files participating in client-server transactions. Message-IDs are generated in a manner analogous to Message-IDs used in Internet email and Usenet articles. The Request-ID element is a Message-ID used by the DDF client  34  to uniquely identify a request DDF file being submitted to a server. The response DDF file generated in response to such a request DDF file is guaranteed to contain this Request-ID in order to assist the DDF client  34  in associating the response DDF file with the request DDF file. The Response-ID element is a Message-ID used to uniquely identify a response DDF file. Such a Response-ID may be used by the DDF client  34  when submitting future requests for the same resource to the server. 
     Other elements which may be included in a DDF file include, but are not limited to: 
     Last-modified, indicating the time that the referenced document was last modified. The format and meaning of this field are analogous to the HTTP/1.1. 
     Title, indicating a title to associate with the DDF. 
     Date, indicating that time of creation of the DDF. 
     Resource-expires, indicating the time that the referenced document expires on the hosting server. 
     If-modified-since, used in DDF cache validation requests. The interpretation and function of If-modified-since is that same as in HTTP/1.1 
     Note that all dates within DDF elements use the syntax defined in the HTTP/1.1 specification. 
     A DDF does not need to contain all of the information contained within the DDF&#39;s referenced document. Typically a DDF will contain only structural and meta information derived from the referenced document. A DDF can be thought of as a promise of service that manifests itself as the bits of the referenced document only when presented to an appropriate server with a request to produce the referenced document. 
     FIG. 3 shows an approach for applying a transformation method to a DDF file. First, a user obtains a DDF file which will be referred to as the “source” DDF file (step  46 ). For purposes of this discussion, assume that the source DDF file in this case is the APP 1  DDF file  20  shown in FIG.  2 . Also, for purposes of this discussion, assume that the APP 1  DDF file  20  is the DDF file shown in Table 1, in which the referenced document is an Adobe Photoshop document. The user obtains the source DDF file in any of a number of ways, for example, by browsing an online gallery of Photoshop images using a DDF-enabled web browser, selecting one of the Photoshop images, and selecting a “Save to DDF” menu item. The user stores APP 1  DDF file  20  on local disk  38  for future use. 
     The user activates DDF client software  34  to generate request DDF file  42 , encapsulating the source DDF file and a request to transform the source DDF file&#39;s referenced document into GIF format (step  48 ). In general, to apply a method within a DDF file to the DDF file&#39;s referenced document, a request DDF file containing an Expose-information element or an Apply-transformation element is produced. These are possibly empty elements that declare calls to an applicable Transformation or Information method declared by the DDF file. These elements are further qualified by appropriate attributes taken from the attribute list of the Transformation or Information method being applied. The DDF itself is an implicit first argument to the method. Additional arguments, if present, are encoded as the contents of the Arguments element of the enclosing DDF. For example, a minimal Apply-transformation element to convert a file into GIF format could appear in a DDF file as &lt;Apply-transformation NAME=“convertToGIF”/&gt;. This Apply-transformation element would apply the method named “convertToGIF” to the DDF containing the Apply-transformation element. 
     In the case of a request to transform the Photoshop document described by the DDF file of Table 1, the request takes the form of an Apply-transformation element in the request DDF file  42 . The request DDF file  42  encapsulates, within the Source-DDF element, the source DDF to which the Transformation method is to be applied. The source DDF may be incorporated within the Source-DDF element either by value or by reference. Incorporation by value involves copying the entire source DDF into the Source-DDF element. Incorporation by reference involves placing a URL, which points to the source DDF, within the Source-DDF element. The contents of the request DDF file  42  are shown in Table 2. 
     
       
         
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
             
               
                   
                 &lt;DDF&gt; 
               
               
                   
                  &lt;Date VALUE=“Mon, 28 Jul 1997 20:01:12 GMT”/&gt; 
               
               
                   
                  &lt;Creating-Application VALUE=“DDF Client”/&gt; 
               
               
                   
                  &lt;Apply-Transformation NAME=“PhotoshopToGif” 
               
               
                   
                   Provider = “http://ddf.company.com/ptg.class”/&gt; 
               
               
                   
                  &lt;Source -DDF&gt; 
               
               
                   
                   &lt;!-- DDF of Table 1 is embedded by value here --&gt; 
               
               
                   
                  &lt;/Source-DDF&gt; 
               
               
                   
                 &lt;/DDF&gt; 
               
               
                   
                   
               
             
          
         
       
     
     The client transmits the request DDF file  42  to the location of the method provider indicated by the PROVIDER attribute of the appropriate Apply-transformation or Apply-information element of the request DDF file  42  (step  50 ). For example, referring to FIG. 1, the client  30  transmits the request DDF file  42  to application APP 1  DDF servlet  26  over network  28 . The APP 1  DDF servlet  26  is a provider of a method to transform Photoshop files into GIF files. The method provider applies the requested transformation to the source DDF file&#39;s referenced document and produces a transformed file, which it stores locally (step  52 ). For example, referring to FIG. 1, the APP 1  DDF servlet  26  transforms native file  18  from Photoshop format into a GIF file  22 , stored on disk  16 . 
     The method provider produces a response DDF file containing information about the transformed file, including a URL pointing to the location at which the transformed file is stored (step  54 ). In the case of FIG. 1, for example, APP 1  DDF servlet  26  produces a response DDF file  44  (FIG. 2) containing a URL pointing to the GIF file  22 . The response DDF file  44  is shown in Table 3. 
     
       
         
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
             
               
                   
                 &lt;DDF&gt; 
               
               
                   
                  &lt;Date VALUE=“Mon, 28 Jul 1997 20:06:12 GMT”/&gt; 
               
               
                   
                  &lt;Last-Modified VALUE=“Mon, 28 Jul 1997 20:05:12 GMT”/&gt; 
               
               
                   
                  &lt;URL VALUE=“http://ddf.company.com/house.gif”/&gt; 
               
               
                   
                  &lt;TITLE VALUE=“Picture of House”/&gt; 
               
               
                   
                  &lt;Content-Type VALUE=“Image/Gif”/&gt; 
               
               
                   
                  &lt;Content-Length VALUE=“55174”/&gt; 
               
               
                   
                  &lt;Creating-Application VALUE=“Photoshop Server”/&gt; 
               
               
                   
                  &lt;Source-DDF&gt; 
               
               
                   
                   &lt;!-- DDF of Table 2 embedded by value here --&gt; 
               
               
                   
                  &lt;/Source-DDF&gt; 
               
               
                   
                 &lt;/DDF&gt; 
               
               
                   
                   
               
             
          
         
       
     
     The method provider transmits the response DDF file  44  to the client  30  (step  56 ). The client  30  extracts the URL from the response DDF file  44  to request and obtain the transformed file (step  58 ). For example, referring to FIG. 1, the client  30  extracts the URL from the response DDF file  44  to request and obtain the GIF file  22  using standard web browser  36 . Alternatively, in step  56 , the method provider transmits the transformed file directly to the client  30  in order to eliminate an additional client-server transaction. 
     Subsequent requests to the same method provider by the same or a different client for a transformation of the same referenced document into GIF format may be satisfied by the method provider without performing step  52 , because the transformed GIF file may already be accessible to the method provider from a previous transformation. The method provider may also obtain an existing GIF file from some other location. The method used by the method provider to obtain the transformed file in any particular case is transparent to the client. 
     The application-data field of the response DDF can be used to cache the results of applying a method to a source DDF. In the example above involving transforming a Photoshop file into a GIF file, the resulting GIF file is stored within the response DDF&#39;s Application-data element. Subsequent requests by the user for a transformation of the same Photoshop file to GIF format are satisfied without accessing the server, because the GIF file being cached within the Application-data element of the response DDF can be extracted by DDF client software  25  and returned directly to the user. 
     As shown in Table 3, a response DDF encapsulates, within the Source-DDF element, the source DDF to which the response DDF is a response. The source DDF may be incorporated within the Source-DDF element either by value or by reference. Embedding the source DDF within the response DDF (either by reference or by value) provides an audit trail of DDF transactions. 
     Other operations may also be performed on DDFs. Consider a user who queries a database of graphical images. The user picks three images out of a set of the ten images that result from the query. Information about these three images is assembled to construct a single composite DDF file that is either saved on the user&#39;s local disk, printed, mailed to another user, or posted on the World Wide Web. Use of a composite DDF file in the place of the original three documents enables the user to move the composite DDF file around as a single file which is much smaller than the combination of the original three documents. The selected image data will only be retrieved from the image database when necessary, e.g., by a printer when the user requests to print the data, or by an email recipient reading the mail message. Furthermore, the user query could return a set of DDFs instead of the actual images, with the image data being retrieved from the image database at a subsequent time if the user so chooses. 
     The single composite DDF file described above may be implemented by using an Aggregation element. An example of such a DDF is shown in Table 4. An Aggregation element works like a virtual paper clip for putting together a sheaf of DDFs. In other words, the Aggregation element is the DDF conjunction operator. The component DDFs making up the aggregation may be embedded either by value or by reference. The meaning of the Aggregation element is that when the DDF client  25  processes the aggregation, all of the aggregation&#39;s components will be processed. 
     
       
         
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
             
               
                   
                 &lt;DDF&gt; 
               
               
                   
                  &lt;Date VALUE=“Mon Aug 4 09:48:55 PDT 1997”/&gt; 
               
               
                   
                  &lt;Title VALUE=“Photo Album”/&gt; 
               
               
                   
                  &lt;content-Type VALUE=“Application/ddf-aggregation”/&gt; 
               
               
                   
                  &lt;Aggregation&gt; 
               
               
                   
                   &lt;DDF REF=“photo-1.ddf”/&gt; 
               
               
                   
                   &lt;DDF REF=“photo-2.ddf”/&gt; 
               
               
                   
                   &lt;DDF REF=“photo-3.ddf”/&gt; 
               
               
                   
                  &lt;/Aggregation&gt; 
               
               
                   
                 &lt;/DDF&gt; 
               
               
                   
                   
               
             
          
         
       
     
     Similarly, the Alternation element can be used within a composite DDF to provide the functionality of a logical disjunction. The Alternation element is used to allow the DDF client  25  to pick one out of a collection of DDFs. The Alternation element itself does not specify which element to pick; the DDF client  25  might choose one of the alternatives in an Alternation based on constraints provided by the environment. For example, consider an image that is to be delivered to the DDF client  25 . The image may have several representations, e.g., a low-resolution representation for quick screen previews, a medium-resolution representation for printing on an inkjet printer, and a high-resolution representation for sending to a high-end imagesetter. A DDF Alternation element may be used to encapsulate each of the representations within a single DDF element. When the DDF file containing the alternation is delivered to the DDF client  25 , the DDF client  25  chooses which one of the DDF&#39;s encapsulated within the Alternation to retrieve, based on the current user environment. 
     The component DDFs of an alternation may be embedded either by value or by reference. The meaning of the Alternation element is that when the DDF client  25  processes the alternation, one and only one of the alternation&#39;s components will be consumed. A DDF file containing an Alternation element is shown in Table 5. Ellipses indicate portions of the DDF omitted for clarity. 
     
       
         
               
               
             
           
               
                   
                 TABLE 5 
               
               
                   
                   
               
             
             
               
                   
                 &lt;DDF&gt; 
               
               
                   
                  ... 
               
               
                   
                  &lt;Alternation&gt; 
               
               
                   
                   &lt;!-- Alternative 1: embed by value --&gt; 
               
               
                   
                    &lt;DDF&gt; 
               
               
                   
                     &lt;!-- text of DDF file goes here --&gt; 
               
               
                   
                    &lt;/DDF&gt; 
               
               
                   
                   &lt;!-- Alternative 2: embed by reference --&gt; 
               
               
                   
                    &lt;DDF ref=“http://ddf.company.com/document.ddf”/&gt; 
               
               
                   
                  &lt;/Alternation&gt; 
               
               
                   
                 &lt;/DDF&gt; 
               
               
                   
                   
               
             
          
         
       
     
     Another useful operation that can be performed using DDFs is document subsetting. Consider the DDF shown in Table 6. The Application-data element of the DDF contains Start-Page, End-Page, and Number-of-Pages elements, containing information about the starting page, ending page, and number of pages of the referenced document, respectively. A user who wishes to print pages  10  through  20  of the referenced document could use the DDF client  25  to produce a secondary DDF file which encapsulates the original DDF file within the Source-DDF element, having a Start-page of  10  and an End-Page of  20 . An example of such a secondary DDF is shown in Table 7. This secondary DDF could be passed to an appropriate application-specific DDF-aware server to obtain a Portable Document Format (PDF) document that has the document data for pages  10  through  20 . Similarly, such a secondary DDF could be passed to a DDF-aware printer  42  that downloads only the minimal amount of document data needed to render the desired pages. In other words, DDF-aware applications can use such a secondary DDF file as a substitute for the referenced document until the document data is actually needed. 
     
       
         
               
               
             
           
               
                   
                 TABLE 6 
               
               
                   
                   
               
             
             
               
                   
                 &lt;DDF&gt; 
               
               
                   
                  &lt;Date VALUE=“Fri, 27 Jun 1997 20:06:12 GMT”/&gt; 
               
               
                   
                  &lt;URL VALUE=“http://www.company.com/thesis.pdf”/&gt; 
               
               
                   
                  &lt;NAME VALUE=“PhD Thesis”/&gt; 
               
               
                   
                  &lt;Content-Type VALUE=“Application/PDF”/&gt; 
               
               
                   
                  &lt;Content-Length VALUE=“1105232”/&gt; 
               
               
                   
                  &lt;Last-Modified VALUE=“Fri, 05 Auq 1994 01:17:21 GMT”/&gt; 
               
               
                   
                  &lt;Creating-Application&gt; VALUE=“Adobe Framemaker”/&gt; 
               
               
                   
                  &lt;Application-Data&gt; 
               
               
                   
                   &lt;Application-Version VALUE=“5.5”/&gt; 
               
               
                   
                   &lt;Sourae-Data&gt; 
               
               
                   
                    &lt;!-- Application-specific data goes here --&gt; 
               
               
                   
                   &lt;/Source-Data&gt; 
               
               
                   
                   &lt;Number-Of-Pages VALUE=“143”/&gt; 
               
               
                   
                   &lt;Start-Page VALUE-“1”/&gt; 
               
               
                   
                   &lt;End-Page VALUE=“143”/&gt; 
               
               
                   
                   &lt;Number-Of-Chapters VALUE=“6”/&gt; 
               
               
                   
                  &lt;/Application-Data&gt; 
               
               
                   
                 &lt;/DDF&gt; 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
             
           
               
                   
                 TABLE 7 
               
               
                   
                   
               
             
             
               
                   
                 &lt;DDF&gt; 
               
               
                   
                  &lt;Date VALUE=“Fri, 27 Jun 1997 20:06:12 GMT”/&gt; 
               
               
                   
                  &lt;URL VALUE=“http://www.company.com/thesis.pdf”/&gt; 
               
               
                   
                  &lt;TITLE VALUE=“PhD Thesis”/&gt; 
               
               
                   
                  &lt;Content-Type VALUE=“Application/PDF”/&gt; 
               
               
                   
                  &lt;Content-Length VALUE=“1105232”/&gt; 
               
               
                   
                  &lt;Last-Modified VALUE=“Fri, 05 Aug 1994 01:17:21 GMT”/&gt; 
               
               
                   
                  &lt;Creating-Application VALUE=“Adobe Framemaker”/&gt; 
               
               
                   
                  &lt;Application-Data&gt; 
               
               
                   
                   &lt;Application-Version VALUE=“5.5”/&gt; 
               
               
                   
                   &lt;Source-Data&gt; 
               
               
                   
                    &lt;!-- Application-specific data goes here --&gt; 
               
               
                   
                   &lt;/Source-Data&gt; 
               
               
                   
                   &lt;Number-Of-Pages VALUE=“143”/&gt; 
               
               
                   
                   &lt;Start-Page VALUE=“10”/&gt; 
               
               
                   
                   &lt;End-Page VALUE=“20”/&gt; 
               
               
                   
                   &lt;Number-Of-Chapters VALUE=“6”/&gt; 
               
               
                   
                  &lt;/Application-Data&gt; 
               
               
                   
                  &lt;Source-DDF&gt; 
               
               
                   
                   &lt;!-- DDF of Table 6 embedded by value here --&gt; 
               
               
                   
                  &lt;/Source-DDF&gt; 
               
               
                   
                 &lt;/DDF&gt; 
               
               
                   
                   
               
             
          
         
       
     
     Referring to FIG. 4, the document description format plugin  24  is implemented in digital electronic circuitry or in computer hardware, firmware, software, or in combinations of them. Apparatus of the invention may be implemented in a computer program product tangibly embodied in a machine-readable storage device for execution by a computer processor; and method steps of the invention may be performed by a computer processor executing a program to perform functions of the invention by operating on input data and generating output. 
     Suitable processors  1080  include, by way of example, both general and special purpose microprocessors. Generally, a processor will receive instructions and data from a read-only memory (ROM)  1120  and/or a random access memory (RAM)  1110  through a CPU bus  300 . A computer can generally also receive programs and data from a storage medium such as an internal disk  1030  operating through a mass storage interface  1040  or a removable disk  1010  operating through an I/O interface  1020 . The flow of data over an I/O bus  1050  to and from I/O devices  1010 ,  1030 ,  1060 ,  1070  and the processor  1080  and memory  1110 ,  1120  is controlled by an I/O controller. User input is obtained through a keyboard  1070 , mouse, stylus, microphone, trackball, touch-sensitive screen, or other input device. These elements will be found in a conventional desktop or workstation computer as well as other computers suitable for executing computer programs implementing the methods described here, which may be used in conjunction with any digital print engine  1075  or marking engine, display monitor  1060 , or other raster output device capable of producing color or gray scale pixels on paper, film, display screen, or other output medium. 
     Storage devices suitable for tangibly embodying computer program instructions include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks  1030  and removable disks  1010 ; magneto-optical disks; and CD-ROM disks. Any of the foregoing may be supplemented by, or incorporated in, specially-designed ASICs (application-specific integrated circuits). 
     Although elements of the invention are described in terms of a software implementation, the invention may be implemented in software or hardware or firmware, or a combination of the three. 
     The present invention has been described in terms of an embodiment. The invention, however, is not limited to the embodiment depicted and described. Rather, the scope of the invention is defined by the claims.