Deriving +from COpenFont

Overview

Write a class derived from COpenFont to +represent an instance of a typeface at a particular size, provide bitmaps +of the glyphs, and determine whether a particular character is defined in +the typeface. An object of this type is created by COpenFontFile::GetNearestFontInPixelsL() when +it is called by the Font and Bitmap server.

To derive from COpenFont:

Write a function to +construct the derived object on the shared heap. The base class constructor +must be called when creating the derived object.
Provide an implementation +for COpenFont's pure virtual function RasterizeL().

These are discussed further in the following sections.

Construction using the shared heap

The derived +object must be created on a shared heap because it is shared by several processes, +e.g. the Font and Bitmap Server and its clients. The following code fragment +shows how the FreeType implementation creates the object on the shared heap aHeap:

CFreeTypeFont* CFreeTypeFont::NewL(RHeap* aHeap, + COpenFontSessionCacheList* aSessionCacheList, + CFreeTypeFontFile* aFontFile,TInt aSizeInPixels) + { + CFreeTypeFont* f = + (CFreeTypeFont*)aHeap->AllocL(sizeof(CFreeTypeFont)); + new(f) CFreeTypeFont(aHeap,aSessionCacheList,aFontFile); + CleanupStack::PushL(f); + f->ConstructL(aSizeInPixels); + CleanupStack::Pop(); + return f; + }

Note that we use aHeap->AllocL() to +obtain memory, then construct in place using a placement argument to new.

The +base class constructor is called when creating the derived object. Pass the +arguments aHeap and aSessionCacheList supplied +to COpenFontFile::GetNearestFontInPixelsL(), and pass the +address of the COpenFontFile object that creates the COpenFont (i.e., +'this') as aFile.

Implementing RasterizeL()

The derived class's RasterizeL() function +is called by the Font and Bitmap Server to return the bitmap for a specified +Unicode character. The function prototype is given below:

virtual void RasterizeL(TInt aCode,TOpenFontGlyphData* aGlyphData) = 0;

Implementations +must create a bitmap, in Symbian's run-length-encoded format, for the Unicode +character aCode. The bitmap must be placed in aGlyphData->iBitmapBuffer, +and the character metrics must be placed in aGlyphData->iMetricsBuffer. +The other parts of aGlyphData should be left alone.

The +actual rasterization task should be passed to the rasterization engine. One +way of doing this is to call some rasterization member function of the COpenFontFile object, +to which a pointer can be obtained by calling COpenFont::File(). +The derived COpenFontFile object can then call a rasterizer +function via a pointer it owns to the rasterizer context object. Note that if the +rasterizer context object is derived from COpenFontRasterizerContext, +it will have utility functions for writing bits to the bitmap buffer in Symbian's +run-length-encoded format.

The FreeType implementation follows this +strategy. CFreeTypeFont::RasterizeL() calls CFreeTypeFontFile::RasterizeL(), +which calls CFreeTypeContext::RasterizeL().

The +run-length-encoded bitmap format

The run-length-encoded format +is a packed binary format starting on a byte boundary and made up of a number +of sections.

Each section starts with a five-bit header. If the first +bit of the header is 0 the next four bits are a repeat count, starting with +the least significant bit, and a single row of bits follows (the number of +bits in a row is specified by aGlyphData->iMetricsBuffer.Width()). +If the first bit of the header is 1 the next four bits are a count of non-repeating +rows, again starting with the least significant bit, and that many rows of +bits follow.

See How +to derive from COpenFontRasterizerContext for an implementation of +this algorithm.