A printer driver provides support for the various fonts provided or supported by GDI.
Some printer drivers use a new format for fonts when running in protected mode (with Windows running in either standard or 386 enhanced mode) on an 80386 or 80486-based computer. This format allows for fonts that can exceed 64K bytes of data, as well as some other options. In this situation, GDI will convert all the 2.x format fonts to the new format so that only one font file format is used in memory.
If a printer driver prints GDI bitmap fonts directly, it should be modified to handle big fonts. In many cases, a driver will not need to be updated if it is a banding monochrome driver that uses only the brute functions. The brute functions are already aware of the new format. For more information about brute functions, see Section 4.1.25 “The Brute Functions,” later in this chapter.
If a printer driver uses the color library, the library supports the big font format, as well as the old 2.x format.
GDI provides TrueType support for any driver that supports GDI raster fonts (that is, it supports drivers that set the DT_RASPRINTER value in the dpTechnology member of the GDIINFO structure). To support TrueType on these printers, GDI generates a raster font from the TrueType outlines and passes the font to the driver just as with any other raster font. This means many Windows 3.0 printer drivers, especially drivers for dot-matrix printers, currently support TrueType—no code changes in the driver are required.
Printer drivers that do not support raster fonts must be modified to support TrueType. Typically, the modification consists of adding code to convert TrueType fonts into a downloadable font format that the printer accepts. To simplify the task of adding TrueType support to nonraster printer drivers, GDI provides several TrueType service functions.
Printer drivers can use TrueType service functions to retrieve information about TrueType fonts, to retrieve bitmaps of individual glyphs in the fonts, and to realize a complete font, retrieving both information and bitmaps. Following are the service functions and brief descriptions.
| Function | Description |
| EngineDeleteFont | Deletes a realized TrueType font. |
| EngineEnumerateFont | Enumerates TrueType fonts. |
| EngineGetCharWidth | Retrieves character widths for TrueType fonts. |
| EngineGetGlyphBmp | Retrieves the bitmap for a rasterized glyph. |
| EngineRealizeFont | Realizes a TrueType font. |
| EngineSetFontContext | Sets the context for rasterizing glyphs. |
| GetRasterizerCaps | Specifies whether TrueType is available. |
A printer driver can determine whether TrueType is currently available by calling the GetRasterizerCaps function. Since users can turn TrueType off, it is important that a driver check for TrueType before generating output.
If TrueType is on, the driver can call the EngineEnumFonts function whenever GDI calls the driver's EnumDFonts function. This gives the font engine the chance to enumerate the TrueType fonts having the specified font. The driver can call the EngineRealizeFont function whenever GDI calls the driver's RealizeObject function. This gives the TrueType font engine an opportunity to generate a physical font that matches the specified logical font. When the engine realizes a font, it fills a FONTINFO structure with information about the font as well as the bitmap data for the individual glyphs. A driver can use this information to create a downloadable font in the format recognized by the printer.
In general, a printer driver should call the EngineRealizeFont and EngineEnumerateFont functions before processing its own device fonts (if any). If GDI requests that the driver delete a realized TrueType font (by calling RealizeObject), the driver must call the EngineDeleteFont function to delete the font.
A printer driver can retrieve character width information for a realized TrueType font by using the EngineGetCharWidth function. It can retrieve bitmaps for individual glyphs in the font by using the EngineSetFontContext and EngineGetGlyphBmp functions. The driver must call EngineSetFontContext first to set the font before calling EngineGetGlyphBmp. Drivers for printers that accept individual glyph definitions (as opposed to full font definitions) can use the character width information and bitmap data to download individual glyphs.
The spot size is a set of values that help the TrueType rasterizer create the best glyph bitmaps for a given printer. All printer drivers can benefit by setting the spot size in their GDIINFO structure regardless of whether the driver uses TrueType service functions. The spot size should be specified in the dpSpotSizeX and dpSpotSizeY members of the driver's GDIINFO structure. If these members are not zero, GDI passes the values to the TrueType engine to help it rasterize glyphs.