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1.3.1 Metafile Structure

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EMF+ defines a set of graphical images and text using commands, objects, and properties similar to Windows GDI+ [MSDN-GDI+]. EMF+ metafiles are portable, device-independent containers for graphical images, and they are used for sending commands and objects to output devices, such as displays and printers, which support the drawing of images and text. The device or media that receives such a metafile can be located on a network, or it can be connected directly to the computer running the operating system on which the metafile is created.

EMF+ metafiles are actually a form of EMF metafile in which EMF+ records (section 2.3) are embedded in EMF records ([MS-EMF] section 2.3). Embedding EMF+ records in EMF metafiles is possible because of the EMF capability to embed arbitrary private data in certain types of records. This is illustrated by the figure that follows. Note that multiple EMF+ records can be embedded in a single EMF record.

The EMF record in which arbitrary private data can be embedded is called an EMF comment record (EMR_COMMENT_EMFPLUS, [MS-EMF] section 2.3.3.2).

As shown in the following figure, the first EMF+ record in the metafile, the EMF+ header record (EmfPlusHeader, section 2.3.3.3), is embedded within the first EMF record following the EMF header record (EMR_HEADER, [MS-EMF] section 2.3.4.2); and the last EMF+ record, the EMF+ end of file record (EmfPlusEndOfFile, section 2.3.3.1), is embedded within the EMF record immediately preceding the EMF end of file record (EMR_EOF, [MS-EMF] section 2.3.4.1).

EMF+ metafiles

Figure 1: EMF+ metafiles

In the EMF metafile in this figure, Case I shows a group of EMF+ records, followed by some EMF records, followed by another group of EMF+ records; and Case II shows a group of EMF+ records terminated by an EMF+ get device context record (EmfPlusGetDC, section 2.3.3.2), followed by some EMF records, followed by another group of EMF+ records. The presence or absence of the get device context record can determine how the metafile is processed.

The structure of an EMF+ metafile is such that EMF+ records are embedded in EMF records, meaning that the following EMF records are always present:

  • EMF header: Required for all types of metafiles

  • EMF comment: Required to contain EMF+ records

  • EMF end of file: Required for all types of metafiles

The EMF+ header record contains flags that distinguish between the following two different types of EMF+ metafiles.<1>

  • Metafiles identified as EMF+ Only can contain both EMF+ records and EMF records. All EMF+ records are used to render the image. The EMF records that are part of the drawing are those preceded by an EMF+ get device context record—case II in the figure above. EMF record processing stops when the next EMF+ record of any type is encountered.

    If a system that cannot play back EMF+ records attempts to play the metafile by using only EMF records, the drawing might be incomplete. If a system performs EMF+ playback mode as expected, then no EMF drawing records are processed unless they are preceded by an EMF+ get device context record. For example, in case I, an EMF+ Only playback mode would process the EMF control records and none of the EMF drawing records. As a result, EMF records alone are not sufficient to render a drawing that was recorded in an EMF+ Only metafile.

  • Metafiles identified as EMF+ Dual can also contain both EMF+ records and EMF records. The EMF+ Dual flag indicates that the metafile contains a complete set of EMF records sufficient to correctly render the entire drawing on a system that cannot playback EMF+ records. This feature makes it possible to render an image with different levels of graphics support in the operating system. However, only one or the other type of records is processed. All records are enumerated sequentially. For EMF playback, the metafile player only uses EMF records and ignores EMF+ records. For EMF+ playback, the metafile is played as if it is EMF+ Only.

Regardless of the EMF+ Dual flag setting, EMF records that follow an EMF+ get device context record are played, until the next EMF+ record, EMF header record, or EMF end of file record.

Note: EMF+ is not considered an extension to the EMF feature set; that is, EMF+ does not define new EMF records. EMF+ is semantically a completely separate, independent format. EMF+ records define graphical images and text using commands, objects, and properties of GDI+.