Creating +a Font Rasterizer Plug-in

Overview

The steps required to write a font rasterizer +DLL are:

Implement an exported NewL() factory +function to create rasterizer objects (which you derive from COpenFontRasterizer). +You may choose to implement this as part of your rasterizer object.
Write a class derived +from COpenFontRasterizer. This will need to store any context +associated with the rasterization process rather than with individual typefaces. +It also implements a function to create your COpenFontFile derived +objects.
Write a class derived +from COpenFontFile. This class manages a single file representing +a typeface.
Write a class derived +from COpenFont.
Place all the code into +a DLL of the right type and provide an exported function to create the rasterizer +object.

Deriving from COpenFontRasterizer, COpenFontFile and COpenFont are +discussed in the "See also" topics below. This section describes the operations +required to package the code as a polymorphic DLL.

For more information, +see:

Deriving +from COpenFontRasterizer
Deriving +from COpenFontFile
Deriving +from COpenFont
Deriving +from COpenFontRasterizerContext

The DLL

A font rasterizer DLL is an ordinary Symbian +polymorphic DLL. It exports just one function and has no unusual behavior.

In +particular, it must have no writable static data. This is an important constraint +on the use of code from outside sources. See Static +Data for more information.

The exported +function

The exported function is a factory function that creates +objects of a class derived from COpenFontRasterizer. It will +be called just once — when the Font and Bitmap Server loads the DLL.

Although +the name of the function does not matter because it is loaded by ordinal, +implementers should follow the practice of making it a static member of the +rasterizer class, called NewL(). Therefore, if the rasterizer +class is CExampleRasterizer (derived from COpenFontRasterizer), +the exported function is:

static COpenFontRasterizer* CExampleRasterizer::NewL()

The function must take no arguments. It must create a COpenFontRasterizer derived +object on the heap, returning it to the caller. The caller is responsible +for deleting the object. As the name of the example function suggests, it +is allowed to leave.

Example

The exported function +used in the FreeType rasterizer is given below:

EXPORT_C COpenFontRasterizer* CFreeTypeRasterizer::NewL() + { + CFreeTypeRasterizer* r = new(ELeave) CFreeTypeRasterizer; + CleanupStack::PushL( r ); + r->iContext = CFreeTypeContext::NewL(); + CleanupStack::Pop(); + return r; + } +

Project file

The name of the rasterizer DLL, its +type, and its UIDs are all specified in the MMP file. Note that:

All capabilities except +TCB (Trusted Computing Base) are required.
The targettype must +be PLUGIN .
The UID line +identifies the project's UID2 and UID3 values. UID2 must be 0x10009D8D, +which identifies the DLL as an ECOM plug-in. UID3 must be 0x100012A7, +which identifies the ECOM plug-in as a font rasterizer.

Example

A fragment of the project file for the freetype +DLL is given below.

target freetype.dll +CAPABILITY All -Tcb +targettype PLUGIN +UID 0x10003B1F 0x100012A7

Header files

Font rasterizer DLLs must #include the +following header files, which are located in the standard include directory \epoc32\include\:

openfont.h for +the Font Store classes
e32uid.h for +the UID value for a dynamic library
f32file.h for +the file handling classes