Windows Metafile (WMF) is an image file format originally designed for Microsoft Windows in the 1990s. The original Windows Metafile format was not device-independent (though could be made more so with placement headers) and may contain both vector graphics and bitmap components. It acts in a similar manner to SVG files. WMF files were later superseded by Enhanced Metafiles (EMF files) which did provide for device-independence. EMF files were then themselves enhanced via EMF+ files.

Essentially, a metafile stores a list of records consisting of drawing commands, property definitions and graphics objects to display an image on screen.^[1] The drawing commands used are closely related to the commands of the Graphics Device Interface (GDI) API used for drawing in Microsoft Windows.

There are three major types of metafiles – a WMF is a 16-bit format introduced in Windows 3.0. It is the native vector format for Microsoft Office applications such as Word, PowerPoint, and Publisher. As of 2023^[update], revision 17 of the Windows Metafile Format specification is available.^[2] EMF files, which replaced WMF files, work on the same principle only it is a 32-bit file format that also allows for the embedding of private data within "comment" records.^[3] EMF+ is an extension to EMF files and embedded in these comment records, allowing for images and text using commands, objects and properties that are similar to Windows GDI+.^[4]

The original 16 bit WMF file format was fully specified in volume 4 of the 1992 Windows 3.1 SDK documentation^[5] (at least if combined with the descriptions of the individual functions and structures in the other volumes), but that specification was vague about a few details. These manuals were published as printed books available in bookstores with no click through EULA or other unusual licensing restrictions (just a general warning that if purchased as part of a software bundle, the software would be subject to one).

Over time the existence of that historic specification was largely forgotten and some alternative implementations resorted to reverse engineering to figure out the file format from existing WMF files, which was difficult and error prone.^[6] In September 2006, Microsoft again published the WMF file format specification in a more complete form^[7] in the context of the Microsoft Open Specification Promise, promising to not assert patent rights to file format implementors.^[8]

Microsoft later deprecated WMF files in favour of 32-bit EMF files as WMF files had real issues with device independence, despite the use of a "placeable" file header which provided basic device independence. Microsoft found that developers who use the format were "[embedding] application, location, or scaling comments in the metafiles... Others added headers to the metafile that provided various application-specific information", causing major compatibility issues.^[9] Thus, in 1992 with Windows NT 3.1, Microsoft introduced the Enhanced Metafile format (EMF)^[10] — a format which was based on the Win32 API and with which they built-in device independence.^[11][9] — these were also known as NT metafiles.^[12] With the release of Windows XP and GDI+, the set of records had to be significantly increased and so Microsoft released EMF+ as an extension to the existing EMF file format.^[10][13]

WMF, EMF and EMF+ files all consist of a series of records that are played back to produce graphical output. Some records define objects which can specify graphical objects used to determine how graphics should be drawn (e.g. pens specify the color and width of lines). Each of these objects are stored in metafiles and are placed into an object table, which tracks the usage of graphic objects while processing the metafile. The object table is an associative array of indexes to graphical object structures defined within the metafile.

WMF and EMF files handle object processing differently to EMF+ records in EMF files. As a WMF and EMF file is being processed, the records are read into an object table once an object is defined. If an object is deleted then the object is released from the table and the identifier can be reused. Notably an object will not be used until it is specifically selected during record playback.^[14][15] This differs for EMF+ files, which also use an associative array via a hashmap which records the object along with an object identifier. However, unlike WMF and EMF files which can delete an object, when a new object is created that has the same index as an existing object, the entry in the table is replaced with the new object. An EMF file also does not need to specifically select an object before it is used.^[16]

The WMF format was designed to be executed by the Windows GDI layer in order to restore the image, but as the WMF binary files contain the definition of the GDI graphic primitives that constitute this image, it is possible to design alternative libraries that render WMF binary files or convert them into other graphic formats.

• PostScript
• Vector Markup Language (VML)
• Scalable Vector Graphics (SVG)