About Metafiles

A metafile is a collection of structures that store a picture in a device-independent format. Device independence is the one feature that sets metafiles apart from bitmaps: unlike a bitmap, a metafile guarantees device independence. For example, when an application creates a picture measuring 2 by 2 inches on a VGA display and stores that picture in a metafile, the picture maintains its original dimensions when printed on a 300 dpi laser printer or copied over a network and displayed in another application that is running on an 8514/A video display. There is a drawback to metafiles, however; they are generally drawn more slowly than bitmaps. Therefore, if an application requires fast drawing and device independence is not an issue, it should use bitmaps instead of metafiles.

Internally, a metafile is an array of variable-length structures called metafile records. The first records in the metafile specify general information such as the resolution of the device on which the picture was created, the dimensions of the picture, and so on. The remaining records, which constitute the bulk of any metafile, correspond to the graphics device interface (GDI) functions required to draw the picture. These records are stored in the metafile after a special metafile device context (DC) is created. This DC is then used for all drawing operations required to create the picture. When Windows processes a GDI function associated with a metafile DC, it converts the function into the appropriate data and stores this data in a record appended to the metafile.

After a picture is complete and the last record is stored in the metafile, the metafile can be passed to another application by means of the clipboard, embedded within another file, stored on disk, or played repeatedly. A metafile is played when its records are converted to device commands and processed by the appropriate device.

There are two types of metafiles: enhanced and Windows. An enhanced metafile is used in applications written using the Microsoft® Win32® application programming interface (API). The enhanced format consists of a header, a table of handles to GDI objects, a private palette, and an array of metafile records. Enhanced metafiles provide true device independence. (You can think of the picture stored in an enhanced metafile as a "snapshot" of the video display taken at a particular moment. This "snapshot" maintains its dimensions no matter where it appears: on a printer, a plotter, or the desktop, or in the client area of any Win32-based application.)

A Windows metafile is used for applications written using the Windows version 3.x API. The Windows format consists of a header and an array of metafile records. Windows-format metafiles are limited in their capabilities and should rarely be used the Windows-metafile API is supported to maintain backward compatibility with applications that were written to run with Windows version 3.x.

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