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<concept id="GUID-5A89E3C4-DEE7-5823-A109-92ED0D87B58C" xml:lang="en"><title>Deriving

from COpenFont</title><prolog><metadata><keywords/></metadata></prolog><conbody>

<section><title>Overview</title> <p>Write a class derived from <codeph>COpenFont</codeph> 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 <codeph>COpenFontFile::GetNearestFontInPixelsL()</codeph> when

it is called by the Font and Bitmap server. </p> <p>To derive from <codeph>COpenFont</codeph>: </p> <ul>

<li id="GUID-855B3E95-D4FB-514D-9C9A-E325D4572F8B"><p>Write a function to

construct the derived object on the shared heap. The base class constructor

must be called when creating the derived object. </p> </li>

<li id="GUID-1B4CA215-8150-580D-A9F2-3E7000A3317C"><p>Provide an implementation

for <codeph>COpenFont</codeph>'s pure virtual function <codeph>RasterizeL()</codeph>. </p> </li>

</ul> <p>These are discussed further in the following sections. </p> </section>

<section><title>Construction using the shared heap </title> <p>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 <codeph>aHeap</codeph>: </p> <codeblock id="GUID-BF5816D4-8EFA-5A63-860F-F6F42A49914D" xml:space="preserve">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;

    }</codeblock> <p>Note that we use <codeph>aHeap-&gt;AllocL()</codeph> to

obtain memory, then construct in place using a placement argument to <codeph>new</codeph>. </p> <p>The

base class constructor is called when creating the derived object. Pass the

arguments <codeph>aHeap</codeph> and <codeph>aSessionCacheList</codeph> supplied

to <codeph>COpenFontFile::GetNearestFontInPixelsL()</codeph>, and pass the

address of the <codeph>COpenFontFile</codeph> object that creates the <codeph>COpenFont</codeph> (i.e.,

'this') as <codeph>aFile</codeph>. </p> </section>

<section><title>Implementing RasterizeL()</title> <p>The derived class's <codeph>RasterizeL()</codeph> function

is called by the Font and Bitmap Server to return the bitmap for a specified

Unicode character. The function prototype is given below: </p> <codeblock id="GUID-806BC2E7-F34E-5809-BE79-56319147D77B" xml:space="preserve">virtual void RasterizeL(TInt aCode,TOpenFontGlyphData* aGlyphData) = 0;</codeblock> <p> Implementations

must create a bitmap, in Symbian's run-length-encoded format, for the Unicode

character <codeph>aCode</codeph>. The bitmap must be placed in <codeph>aGlyphData-&gt;iBitmapBuffer</codeph>,

and the character metrics must be placed in <codeph>aGlyphData-&gt;iMetricsBuffer</codeph>.

The other parts of <codeph>aGlyphData</codeph> should be left alone. </p> <p>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 <codeph>COpenFontFile</codeph> object,

to which a pointer can be obtained by calling <codeph>COpenFont::File()</codeph>.

The derived <codeph>COpenFontFile</codeph> object can then call a rasterizer

function via a pointer it owns to the rasterizer context object. Note that <i>if</i> the

rasterizer context object is derived from <codeph>COpenFontRasterizerContext</codeph>,

it will have utility functions for writing bits to the bitmap buffer in Symbian's

run-length-encoded format. </p> <p>The FreeType implementation follows this

strategy. <codeph>CFreeTypeFont::RasterizeL()</codeph> calls <codeph>CFreeTypeFontFile::RasterizeL()</codeph>,

which calls <codeph>CFreeTypeContext::RasterizeL()</codeph>. </p> <p><b>The

run-length-encoded bitmap format</b> </p> <p>The run-length-encoded format

is a packed binary format starting on a byte boundary and made up of a number

of sections. </p> <p>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 <codeph>aGlyphData-&gt;iMetricsBuffer.Width()</codeph>).

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. </p> <p>See <xref href="GUID-F44D974F-AADE-5E6C-8D69-8D3EEE4BD395.dita">How

to derive from COpenFontRasterizerContext</xref> for an implementation of

this algorithm. </p> </section>

</conbody><related-links>

<link href="GUID-F477E82D-2929-5C69-BF6D-A69A61AC9EA5.dita"><linktext>Creating

a Font Rasterizer Plug-in</linktext></link>

<link href="GUID-1CF78E25-8C83-5812-AE6F-8612B3F9CDA9.dita"><linktext>Deriving

from COpenFontRasterizer</linktext></link>

<link href="GUID-BCF6FEF0-1792-5DEA-A2D7-896E47163547.dita"><linktext>Deriving

from COpenFontFile</linktext></link>

<link href="GUID-F44D974F-AADE-5E6C-8D69-8D3EEE4BD395.dita"><linktext>Deriving

from COpenFontRasterizerContext</linktext></link>

</related-links></concept>