Filename extension .wmf, .emf, .wmz, .emz Developed by Microsoft | Internet media type image/wmf, image/emf Type of format Image file formats | |
Initial release 1990; 27 years ago (1990) Latest release 12
(30 June 2015; 20 months ago (2015-06-30)) | ||
Windows Metafile (WMF) is an image file format originally designed for Microsoft Windows in the 1990s. Windows Metafiles are intended to be portable between applications and may contain both vector graphics and bitmap components. It acts in a similar manner to SVG files.
Contents
Essentially, a WMF file stores a list of function calls that have to be issued to the Windows Graphics Device Interface (GDI) layer to display an image on screen. Since some GDI functions accept pointers to callback functions for error handling, a WMF file may erroneously include executable code.
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 2015 revision 12 of the Windows Metafile Format specification is available for online reading or download as PDF.
Specifications and patents
The original 16 bit WMF file format was fully specified in volume 4 of the 1992 Windows 3.1 SDK documentation (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. In September 2006, Microsoft again published the WMF file format specification in the context of the Microsoft Open Specification Promise, promising to not assert patent rights to file format implementors.
Variants
In 1993, the 32-bit version of Win32/GDI introduced the Enhanced Metafile (EMF), a newer version with additional commands. EMF is also used as a graphics language for printer drivers. Microsoft recommends that "Windows-format" (WMF) functions only "rarely" be used and "enhanced-format" (EMF) functions be used instead.
With the release of Windows XP, the Enhanced Metafile Format Plus Extensions (EMF+) format was introduced. EMF+ provides a way to serialize calls to the GDI+ API in the same way that WMF/EMF stores calls to GDI.
There are also compressed versions of Windows Metafiles known as Compressed Windows Metafile (WMZ) and Compressed Windows Enhanced Metafile (EMZ), which are basically gzip compressed WMF and EMF files correspondingly.
Vulnerabilities
Over the years several vulnerabilities in Windows Metafile handling were reported.
Implementations
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. For example, the Batik library is able to render WMF files and convert them to their Scalable Vector Graphics (SVG) equivalent. The Vector Graphics package of the FreeHEP Java library allows the saving of Java2D drawings as Enhanced Metafiles (EMF). Inkscape and XnView can export to WMF or EMF.