The Open Packaging Conventions (OPC) is a container-file technology initially created by

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to store a combination of

XML Extensible Markup Language (XML) is a markup language and file format for storing, transmitting, and reconstructing arbitrary data. It defines a set of rules for encoding documents in a format that is both human-readable and machine-readable. ...

and non-XML files that together form a single entity such as an

Open XML Paper Specification Open XML Paper Specification (also referred to as OpenXPS) is an open specification for a page description language and a fixed-document format. Microsoft developed it as the XML Paper Specification (XPS). In June 2009, Ecma International adop ...

(OpenXPS) document. OPC-based file formats combine the advantages of leaving the independent file entities embedded in the document intact and resulting in much smaller files compared to normal use of XML.

Specifications

The OPC is specified in Part 2 of the

Office Open XML Office Open XML (also informally known as OOXML) is a zipped, XML-based file format developed by Microsoft for representing spreadsheets, charts, presentations and word processing documents. Ecma International standardized the initial versi ...

standards ISO/ IEC 29500:2008 and ECMA-376.ISO/IEC 29500-2:2008 - Information technology -- Document description and processing languages -- Office Open XML File Formats -- Part 2: Open Packaging Conventions
ISO The ISO/IEC 29500-2:2008 specification and the second edition of ECMA-376 makes a normative reference to

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's ''.ZIP File Format Specification'' version 6.2.0 (2004), and supplements it with a normative set of clarifications. Note: The older first edition of ECMA-376 makes an informative (''i.e.'', non-normative) reference to the newer PKWARE Inc's ".ZIP File Format Specification" version 6.2.1 (2005). The ZIP format is not specified by any international standard but has widespread community and developer acceptance. Microsoft submitted a draft in 2006 to the

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for a "pack"

URI Scheme A Uniform Resource Identifier (URI) is a unique sequence of characters that identifies a logical or physical resource used by web technologies. URIs may be used to identify anything, including real-world objects, such as people and places, conc ...

(pack://) to be used for URI references to OPC-based packages. The draft expired in 2009, the specified syntax is incompatible with the

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for URI schemes (STD 66, RFC 3986). The scheme is now listed as ''historical''. The ISO 19165:1-2018 recommends the use of the Open Packaging Conventions to implement the Geospatial Package defined in the

Open Archival Information System An Open Archival Information System (or OAIS) is an archive, consisting of an organization of people and systems, that has accepted the responsibility to preserve information and make it available for a Designated Community. The OAIS model can ...

Usage

Both the XML Paper Specification (XPS) and

(OOXML) use Open Packaging Conventions (OPC), which provide a profile of the common ZIP format. In addition to data and document content in XML markup, files in the ZIP package can include other text and binary files in formats such as PNG, BMP, AVI,

PDF Portable Document Format (PDF), standardized as ISO 32000, is a file format developed by Adobe in 1992 to present documents, including text formatting and images, in a manner independent of application software, hardware, and operating systems. ...

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, or even an already packaged ODF file. OPC also defines some naming conventions and an indirection method to allow position independence of binary and XML files in the ZIP archive. OPC files can be opened using common ZIP utilities. OPC allow indirection, chunking and relative indirection.

File formats using the OPC

The OPC is the foundation technology for many new file formats:Adventures in Packaging - Episode 1
May 18, 2009, by Jack Davis, Microsoft Packaging Team Blog: Open Packaging Conventions

Programming

OPC is natively supported in Microsoft

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3.0 by the namespace. Open source libraries exist for other languages. Since

Windows 7 Windows 7 is a major release of the Windows NT operating system developed by Microsoft. It was released to manufacturing on July 22, 2009, and became generally available on October 22, 2009. It is the successor to Windows Vista, released nearl ...

, OPC is also natively supported in the

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through a set of COM interfaces, collectively referred to a
Packaging API
Alternatively, ZIP libraries can be used to create and open OPC files, as long as the correct files are included in the ZIP and the conventions followed.

Package, parts, and relationships

In OPC terminology, the term ''package'' corresponds to a ZIP archive and the term ''part'' corresponds to a file stored within the ZIP. Every part in a package has a unique URI-compliant part name along with a specified content-type expressed in the form of a

MIME Multipurpose Internet Mail Extensions (MIME) is an Internet standard that extends the format of email messages to support text in character sets other than ASCII, as well as attachments of audio, video, images, and application programs. Messa ...

media type A media type (also known as a MIME type) is a two-part identifier for file formats and format contents transmitted on the Internet. The Internet Assigned Numbers Authority (IANA) is the official authority for the standardization and publication o ...

. A part's content-type explicitly defines the type of data stored in the part and reduces duplication and ambiguity issues inherent with file extensions. OPC packages can also include ''relationships'' that define associations between the package, parts, and external resources. In addition to a hierarchy of directories and parts, OPC packages commonly use ''relationships'' to access content through a

directed graph In mathematics, and more specifically in graph theory, a directed graph (or digraph) is a graph that is made up of a set of vertices connected by directed edges, often called arcs. Definition In formal terms, a directed graph is an ordered pai ...

of relationship associations. Relationships are composed of four elements: :* an identifier (ID) :* an optional source (the package or a part within the package) :* a relationship type (a URI-style expression that defines the type of the relationship) :* a target (a

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to another part within the package or to an external resource) OPC packages can store parts that contain any type of data (text, images, XML, binary, whatever). The extension ".rels", however, is reserved for storing relationships metadata within "/_rels" subfolders. The subfolder name "_rels", the file extension ".rels" within such directory, and the filename " ontent_Typesxml" in any folder are the only three reserved names for files stored in an OPC package. :; / ontent_Typesxml file :: This file defines the

media types for all the parts stored in the package. The "/ ontent_Typesxml" file defines default mappings based on file extensions, along with overrides for specific parts with content-types that are different from the file extension defaults. For example, one of these defined MIME types is: ::: :; /_rels :: The root level "/_rels" folder stores the relationships for the package as a whole. The "/_rels" folder normally contains a file named ".rels". "/_rels/.rels" is an XML file where the starting ''package-level relationships'' are stored. Normally when opening an OPC-based file, applications start by accessing to the "/_rels/.rels" file to read the starting package-level relationships. :; '' artname'.rels :: Each part may have its own relationships. The ''_rels'' folders are where one goes to find the relationships for any given part within the package. To find the relationships for a specific part, one looks in the "_rels" folder that is a sibling of that part: If the part has relationships, the "_rels" folder will contain a file that has one's original part name with a ".rels" appended to it. For example, if the content types part file had any relationships, there would be a file called " ontent_Typesxml.rels" inside the "/_rels" folder. All relationships (including the relations associated to the root package) are represented as XML files. If you open a ".rels" file in a text editor, you can view the actual XML markup that defines all the relationships targeted from that part. A typical relationships file contains XML code like this: which defines two relations for the root package, the first one being considered as the root package (here for an early ''Microsoft XPS'' document, before it was standardized as

within the openxmlformats collection), and the other one being used to reference an alternate form (here a thumbnail rendered image of the first page of the document). The main parts of the embedded documents are often stored within a folder named "/Document" (which may contain subdirectories itself, if the file contains several related documents each of them with various parts), and the optional metadata parts that are not needed for processing the main parts of the document are stored in a folder named "/Metadata"; however these actual folder names are actually specified within the XML-formatted data in "'' artname'.rels" relationship files and the OPC specification allows any folder organisation that is convenient for the application and these two folder names are not required.

Chunking

It encourages documents to be split into small chunks. This is better for reducing the effect of file corruption. And better for data access: for example, all the style information in one XML part, each separate worksheet or table in their own different parts. This allows faster access and less object creation for clients and makes it easier for multiple processes to be working on the same document.

Relative indirection

In the Open Packaging Conventions, each file that has reference has its own ''_rels'' file with the indirection lists. This makes it easier to cut and paste some information with all its associated resources in some cases, provides name scoping to remove the chance of name clashing between files, and so on.

References

External links

Download specification ISO/IEC 29500-2:2012

OPC: A New Standard for Packaging Your Data

Essentials of the Open Packaging Conventions

OPC Digital Signatures: Application Guidelines for Common Criteria Security

Packaging team blog

Open Packaging Conventions (OPC) MSDN Forum

The Addressing Model of the Open Packaging Conventions

OPC implementation test documents

OPC package explorer
to edit XML parts

ISO 19165 Geographic information – Preservation of digital data and metadata – Part 1: Fundamentals {{CAD software Computer file formats Ecma standards IEC standards ISO standards Microsoft initiatives Office Open XML