Формат файла Windows Metafile (WMF): история и особенности

File format for images
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 WMF files were later superseded by Enhanced Metafiles (EMF files) which did provide for device independence.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. 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, revision 17 of the Windows Metafile Format specification is available. 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. 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+.

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 a more complete form in the context of the Microsoft Open Specification Promise, promising to not assert patent rights to file format implementors. 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. Thus, in 1992 with Windows NT 3.1, Microsoft introduced the Enhanced Metafile format (EMF) — a format which was based on the Win32 API and with which they built in device independence. 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.

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. 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.

Bitmap records manage and output bitmap images. Name Description META BITBLT Specifies how to do a bit block transfer. These records can specify a bitmap to use as the source, or a region. META DIBBITBLT Specifies how to do a bit block transfer of a device independent bitmap image. These records can specify a bitmap to use as the source, or a region. META DIBSTRETCHBITBLT Specifies how to do a bit block transfer of a device independent bitmap image, but allows for expansion or contraction of the image. These records can specify a bitmap to use as the source, or a region. META SETDIBTODEV Specifies color to set a block of pixels in a device independent bitmap image. These records can specify a bitmap to use as the source, or a region. META STRETCHBLT Specifies how to do a bit block transfer, but allows for expansion or contraction of the image. META STRETCHDIB Specifies how to do a bit block transfer of a device independent bitmap image, but allows for expansion or contraction of the image. These records can specify a bitmap to use as the source, or a region.

Drawing records produce graphics output. Name Description META ARC Draws an elliptical arc. META CHORD Draws a chord. META ELLIPSE Draws an elliptical arc. META EXTFLOODFILL Fills an area with a brush. META EXTTEXTOUT Draw text with font, background color and text color of the playback device context. META FILLREGION Fills a region with a specified brush. META FLOODFILL Fills an area with a brush. META FRAMEREGION Draws a border around a region of the page with a brush. META INVERTREGION Paints a region of the graphic with inverted colors. META LINETO Draws a half open line from the drawing position defined in the playback device context to a specified point. META PAINTREGION Paint a region with the current brush specified in the playback device context. META PATBLT Paint a region with the current brush specified in the playback device context and then combine the brush color and surface color(s) using a raster operation. META PIE Draw a pie shaped wedge bounded by the intersection of an ellipse and two radials — the line is set via the pen set via the playback device context, and the area bounded by the pie shape is filled in by the current brush of the playback device context. META POLYLINE Draws a series of line segnments by connecting the points in a specified array. META POLYGON Paints a polygon consisting of two or more vertices connected by straight lines. The polygon is outlined by using the pen and filled by using the brush and polygon fill mode that are defined in the playback device context. META POLYPOLYGON Paints a series of closed polygons, which may overlap. Each polygon is outlined by using the pen and filled by using the brush and polygon fill mode that are defined in the playback device context. META RECTANGLE Paints a rectangle, which is outlined by using the pen and filled by using the brush and is filled by using the brush that is defined in the playback device context. META ROUNDRECT Paints a rectangle with rounded corners. The rectangle is outlined by using the pen and filled by using the brush and is filled by using the brush that is defined in the playback device context. META SETPIXEL Sets the pixel at the specified coordinates to the specified color. META TEXTOUT Outputs a character string at the specified location by using the font, background color, and text color that are defined in the playback device context.

State records manage the graphics properties of the playback device context. Name Description META ANIMATEPALETTE Redefines entries in the logical palette that is defined in the playback device context with a specified Palette Object. META EXCLUDECLIPRECT Sets the clipping region that is defined in the playback device context to the existing clipping region minus a specified rectangle. META INTERSECTCLIPRECT Sets the clipping region that is defined in the playback device context to the intersection of the existing clipping region and a specified rectangle. META MOVETO Sets the output position in the playback device context to a specified point. META OFFSETCLIPRGN Moves the clipping region that is defined in the playback device context by specified offsets. META OFFSETVIEWPORTORG Moves the viewport origin in the playback device context by specified horizontal and vertical offsets. META OFFSETWINDOWORG Moves the output window origin in the playback device context by specified horizontal and vertical offsets. META REALIZEPALETTE Maps entries from the logical palette that is defined in the playback device context to the system palette. META RESIZEPALETTE Redefines the size of the logical palette that is defined in the playback device context. META RESTOREDC Restores the playback device context from a previously saved device context. META SAVEDC Saves the playback device context for later retrieval. META SCALEVIEWPORTEXT Scales the horizontal and vertical extents of the viewport that is defined in the playback device context using the ratios formed by specified multiplicands and divisors. META SCALEWINDOWEXT Scales the horizontal and vertical extents of the output window that is defined in the playback device context using the ratios formed by specified multiplicands and divisors. META SETBKCOLOR Sets the background color in the playback device context to a specified color. META SETBKMODE Sets the background mix mode in the playback device context. META SETLAYOUT Defines the layout orientation in the playback device context. META SETMAPMODE Defines the mapping mode in the playback device context. META SETMAPPERFLAGS Defines the algorithm that the font mapper uses when it maps logical fonts to physical fonts. META SETPALENTRIES Defines RGB color values in a range of entries in the logical palette that is defined in the playback device context. META SETPOLYFILLMODE Defines polygon fill mode in the playback device context for graphics operations that fill polygons. META SETRELABS Unused record. META SETROP2 Defines the foreground raster operation mixing mode in the playback device context. META SETSTRETCHBLTMODE Defines the bitmap stretching mode in the playback device context. META SETTEXTALIGN Defines text alignment values in the playback device context. META SETTEXTCHAREXTRA Defines inter character spacing for text justification in the playback device context. META SETTEXTCOLOR Defines the text foreground color in the playback device context. META SETTEXTJUSTIFICATION Defines the amount of space to add to break characters in a string of justified text. META SETVIEWPORTEXT Defines the horizontal and vertical extents of the viewport in the playback device context. META SETVIEWPORTORG Defines the viewport origin in the playback device context. META SETWINDOWEXT Defines the horizontal and vertical extents of the output window in the playback device context. META SETWINDOWORG Defines the output window origin in the playback device context.

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.

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.