How to write a rasterizer DLL

The steps required to write a rasterizer DLL are:

• Obtain or write code that can read and process font files of the desired format. It is usually more practical to get either freeware or paid-for code, than to write it in-house. It may take over a year to write a rasterizer, even for an expert programmer with the right domain knowledge.

• 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.

Note that only one Open Font System rasterizer has so far been released: FreeType. It uses freeware code from the FreeType project (see http://www.freetype.org) to provide a TrueType rasterizer. Fragments of code given in this document are taken from this implementation.

How to derive from COpenFontRasterizer

How to derive from COpenFontFile

How to derive from COpenFont

How to derive from COpenFontRasterizerContext

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

In particular, it must have no writable static data. This is an important constraint on the use of code from outside sources.

Static data

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(). Thus, 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.

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;
    }

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

• The target output file extension must be .dll. Only rasterizer DLLs with this extenstion are loaded during Font and Bitmap Server startup.

• The targetpath is \system\fonts. This is the location from which rasterizers are automatically loaded during Font and Bitmap Server startup.

• The targettype must be dll. This sets the project's UID1 value, identifying it as a DLL. The value of this UID is KDynamicLibraryUidValue (0x10000079).

• The UID line identifies the project's UID2 and UID3 values. UID2 must identify the file as an Open Font System rasterizer, i.e. be equal to KUidOpenFontRasterizerValue (0x10003B1F).

• UID3 identifies the font format that is supported. Standard values are KUidTrueTypeRasterizerValue (0x100012A7) for TrueType fonts, KUidTrueDocRasterizerValue (0x100012A8) for the TrueDoc format invented by BitStream, Inc., and KUidAdobeType1RasterizerValue (0x100012A9) for the Adobe Type 1 format invented by Adobe, Inc.

  If the DLL is for some other font format a new UID3 must be obtained from Symbian.

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

target freetype.dll
targetpath \system\fonts
targettype dll
UID 0x10003B1F 0x100012A7

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

• openfont.h for the Open Font System classes

• e32uid.h for the UID value for a dynamic library

• f32file.h for the file handling classes