Open Document Format (ODF) is an open, XML-based file format standard for word processing documents, spreadsheets, charts, and presentations. The specification was originally developed by Sun Microsystems, but has been standardized by the Organization for the Advancement of Structured Information Standards (OASIS). ODF version 1.0 has been standardized as ISO/IEC 26300:2006. ODF is the primary format for the OpenOffice.org office suite.
File Extensions
The main file extensions for ODF documents are
- .odt for word processing documents
- .ods for spreadsheet documents
- .odp for presentation documents
- .odb for database documents
- .odg for graphical documents
- .odf for mathematical formulae
ODF also supports template files for each type of document. The 'd' in file extension is replaced by a 't' for template files.
File Structure
An ODF document can be as simple as a single XML file. However, this is rarely practical. The standard specifies that an ODF file can also be stored as a collection of several subdocuments. The latter is the most common implementation.
A packaged ODF file will contain, at a minimum, six files and two directories archived into a modified ZIP file. The structure of the basic package is as follows
|-- META-INF
| `-- manifest.xml
|-- Thumbnails
| `-- thumbnail.png
|-- content.xml
|-- meta.xml
|-- mimetype
|-- settings.xml
`-- styles.xml
Again, this represents a minimal ODF file. The structure can become much more complicated as directories can be added that contain embedded images, macros, and the like.
An important caveat in the structure of the ZIP file is that the first file must be the "mimetype" file and it must not be compressed. 1 The string "mimetype" should appear at position 30 and the actual MIME type at position 38. This adaptation makes it possible for operating systems to determine the MIME type of a file without relying on the file extension.
Main Sub-Files
The manifest.xml file contains a list of all files in the packages, as well as their media type, path, and any information required for decryption. The content.xml file contains the content of the document (e.g., the text in a word processing document), while the styles.xml file contains the information on how the content is to be styled. The settings.xml file is self-explanatory.
Metadata
Because ODF files are basically ZIP files, the files contain the same meta-information about each file as that of a standard ZIP archive, namely the name and size of each sub-file, compression information, and creation date of each sub-file. In addition, much metadata is contained within the xml files themselves. The meta.xml file contains metadata for the entire document. The types of metadata contained in the file can comprise pre-defined metadata, user defined metadata, as well as custom metadata:
- which version of ODF is used by the document
- the document generator, that is, the user-agent software that generated or last modified the ODF document. This string is similar to the HTTP user agent string as described in RFC-2616. This can contains the name and version of the software as well as the name of the operating system.
- document title
- document description
- document subject
- keywords
- initial creator
- Creator (person who last modified the document)
- printed by
- creation date/time
- modification date/time
- print date/time
- document template, the path of the document template if one was used to generate the current document
- automatic reload
- hyperlink behavior
- language
- number of editing cycles stored as a string. The number is incremented each time the document is saved.
- editing duration -- amount of time spent editing the document. The specification is not clear as to how this value is to be calculated.
- document statistics -- this field varies by file type, but includes information such as page count, object count, paragraph count, cell count, etc.
- user-defined metadata -- allowable types: string, integer, float, boolean
Conforming applications are permitted to store non-standard fields in this file, and the software should preserve any custom fields.
Not all metadata is stored in the meta.xml file. The content.xml file can contain meta-information such as annotations and tracked changes, as well as the creator and creation date time of those annotations or tracked changes.