EMF File Documentation

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Technical details

Feature	Value
Full name	Enhanced Metafile
File extension	.emf
MIME type	image/emf (also image/x-emf)
Format type	Windows GDI vector image (recorded drawing commands)
Developer	Microsoft
Introduced	1993 (Windows NT 3.1)
Specification	MS-EMF (Microsoft Open Specifications)
Open standard	Partial — publicly documented, Microsoft-controlled
Byte order	Little-endian
First record	EMR_HEADER (record type 1) at offset 0
Signature	DWORD 01 00 00 00 at offset 0; “ EMF” (20 45 4D 46) at offset 40
Predecessor	WMF (16-bit Windows Metafile)
Compressed variant	.emz (EMF + GZIP)
Can embed	Raster bitmaps (via StretchDIBits/BitBlt records)
Common source	Office paste/export, Visio, Windows print spool, clipart
Related extensions	.emz, .wmf, .wmz, .svg, .eps
Free tools	IrfanView, XnView, LibreOffice Draw, Inkscape
Spec URL	learn.microsoft.com (MS-EMF)

What is an EMF file?

EMF stands for Enhanced Metafile, Microsoft's vector graphics format, introduced with Windows NT 3.1 in 1993 as the 32-bit successor to the older 16-bit WMF (Windows Metafile). It is documented in Microsoft's open MS-EMF specification. Rather than storing a grid of pixels, an EMF file records a sequence of GDI (Graphics Device Interface) drawing commands — move and line, curves, polygons, text with fonts, fills, and coordinate transforms — so the image is usually a scalable vector drawing: a diagram, chart, logo, or piece of clipart. EMF records can also carry embedded raster bitmaps, so some EMF files are effectively pixel images in a vector wrapper.

Most people meet EMF through Microsoft Office. Two situations dominate. First, a graphic named image001.emf arrives as an email attachment or sits inside a document, and a non-Microsoft viewer cannot display it. Second, you unzip a .docx or .pptx (both are ZIP packages) and find .emf files in the word/media/ or ppt/media/ folder. EMF is also the format Windows uses internally to spool a page to a printer, which is where the format originally earned its place in the operating system.

The EMR_HEADER record: bounds, frame and the signature

An EMF file is a stream of fixed-structure records, and the very first record is always EMR_HEADER. It defines the coordinate space and the file-level bookkeeping for everything that follows. Its opening fields sit at known offsets:

offset  0  : iType      = 0x00000001   EMR_HEADER record type
offset  4  : nSize                     size of this header record
offset  8  : rclBounds  (16 bytes)     bounding rectangle, device units
offset 24  : rclFrame   (16 bytes)     frame rectangle, .01 mm units
offset 40  : dSignature = 0x464D4520   ASCII " EMF"
offset 44  : nVersion                  metafile version (0x00010000)
offset 48  : nBytes                    total size of the metafile
offset 52  : nRecords                  number of records in the file
offset 56  : nHandles                  size of the GDI object handle table

The iType value of 1 marks the record as the header, and the four bytes at offset 40 spell the ASCII string  EMF (with a leading space) — the reliable way to identify the format. rclBounds gives the drawing's extent in device pixels while rclFrame gives it in hundredths of a millimetre, which together let a renderer scale the picture to any output device. nBytes and nRecords tell a reader exactly how much data and how many records to expect, and nHandles sizes the table that will hold the pens, brushes, and fonts created during playback.

The record stream: playing back GDI commands

After the header, EMF is a sequence of records that a reader “plays back” into a device context, exactly as if the original program were redrawing the picture. Every record shares the same two leading fields: a 32-bit iType naming the operation and a 32-bit nSize giving the record's length, so an unknown record can be skipped by advancing nSize bytes. The records fall into a few families:

• Object creation — EMR_CREATEPEN, EMR_CREATEBRUSHINDIRECT, EMR_EXTCREATEFONTINDIRECTW build GDI objects and put them in the handle table.
• Object selection — EMR_SELECTOBJECT makes a created pen, brush, or font current.
• Drawing — EMR_MOVETOEX, EMR_LINETO, EMR_POLYGON16, EMR_EXTTEXTOUTW emit the actual geometry and text.
• State — EMR_SETWORLDTRANSFORM and EMR_SAVEDC/EMR_RESTOREDC change the coordinate transform and save/restore drawing state.
• Raster — EMR_STRETCHDIBITS and EMR_BITBLT blit an embedded bitmap into the picture.

The stream always ends with EMR_EOF, the end-of-metafile record. Because the picture is a program of drawing calls rather than a bitmap, its fidelity on a non-Windows system depends on how faithfully that system reimplements GDI semantics, which is why cross-platform tools sometimes render EMF text or transforms slightly differently.

EMF versus WMF and EMZ

EMF replaced WMF, the 16-bit Windows Metafile from the Windows 3.x era. WMF records use 16-bit coordinates and a smaller command set tied to the old GDI, whereas EMF uses 32-bit coordinates, adds many records (world transforms, wide-character text, richer paths), and is device-independent. Converting EMF to WMF is therefore a downgrade that can lose features, and is rarely wanted; the reverse (WMF to EMF) is the natural direction.

An .emz file is simply an EMF compressed with GZIP — the relationship is identical to SVGZ versus SVG. Decompressing an .emz with 7-Zip or gunzip yields a plain .emf byte-for-byte, and both hold the same vector drawing. That is why an EMZ magic number is the GZIP 1F 8B rather than the EMF header, and why any EMF tool works on an EMZ once it is decompressed.

Converting EMF to PNG or SVG

The two conversions that matter are to a raster image for the web and to another vector format for editing. For PNG (vector to raster), the quickest Windows route is to open the EMF in IrfanView or XnView and Save As, or to insert it into Word/PowerPoint, right-click, and choose Save as Picture; anywhere, LibreOffice Draw and Inkscape export PNG. Because this rasterises the drawing, pick a resolution high enough for its final use. For SVG (vector to vector), open the EMF in Inkscape or LibreOffice Draw and export SVG; this keeps the paths scalable, though some complex GDI text or transform records may render slightly differently. Prefer PNG over JPG for diagrams and line art, since JPG's lossy compression smears sharp edges.

On macOS and Linux there is no built-in EMF support, so LibreOffice Draw and Inkscape (each with its own EMF interpreter) are the reliable free openers and converters.

References

• Microsoft Open Specifications — MS-EMF: Enhanced Metafile Format
• Microsoft Learn — Metafiles (GDI+)
• The Document Foundation — LibreOffice Draw (opens and converts EMF)

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