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<concept xml:lang="en" id="GUID-F44D974F-AADE-5E6C-8D69-8D3EEE4BD395"><title>Deriving from COpenFontRasterizerContext</title><prolog><metadata><keywords/></metadata></prolog><conbody><section><title>Overview</title> <p>The font rasterizer context provides the link between the framework required by the open font system, and the font rasterizer engine. As such it should contain a function to return a bitmap for the specified character, and then convert it into Symbian's run-length encoded format. Information about this format is provided in "How to derive from <codeph>COpenFont</codeph> ". </p> <p> <codeph>COpenFontRasterizerContext</codeph> is a convenience class — use of this class is optional. It contains a number of utility functions to make it easier to write glyph bitmaps in Symbian's bitmap format. </p> <p>Use <codeph>COpenFontRasterizer::StartGlyph()</codeph> to initialize the buffer for writing the bitmap. Use <codeph>COpenFontRasterizer::WriteGlyphBit()</codeph> to write the individual bits to the buffer. When all the bits have been added to the buffer, call <codeph>COpenFontRasterizer::EndGlyph()</codeph> to write the glyph bitmap to <codeph>aGlyphData-&gt;iBitmapBuffer</codeph>  </p> </section> <section><title>Example</title> <p>The following code fragment shows a partial implementation of <codeph>CFreeTypeContext::RasterizeL()</codeph>. This function creates the glyph bitmap used to represent a specified character, based on the specification in the font file. </p> <p>This fragment has been provided because it shows how the glyph information is obtained from the font file, and how to implement the algorithm to write a glyph in Symbian run-length encoded format. Note the use of <codeph>StartGlyph()</codeph>, <codeph>WriteGlyphBit()</codeph> and <codeph>EndGlyph()</codeph>. </p> <codeblock id="GUID-650B3D46-EFBD-55A2-ABD6-2CAA6A5795D0" xml:space="preserve">//code before here writes the character metrics to aGlyphData

 // Try to use the built in raster map; if it's not big enough use a temporary bigger one.

    TBool use_temp_raster_map = width > KRasterWidth || height > KRasterHeight;

    CRasterMap* cur_raster_map = iRasterMap;

    if (use_temp_raster_map)

        {

        cur_raster_map = CRasterMap::NewL(width,height);

        CleanupStack::PushL(cur_raster_map);

        }

    cur_raster_map->Clear();

    TT_Raster_Map& tt_raster_map = cur_raster_map->RasterMap();

    error = TT_Get_Glyph_Bitmap(aGlyph, &tt_raster_map,-tt_metrics.bearingX,

    (tt_raster_map.rows - ascent) << 6);

    if (error)

        User::Leave(KErrGeneral);

    StartGlyph(aGlyphData);

    const TUint8* p = (const TUint8*)tt_raster_map.bitmap;

    int row = 0;

    while (row < height)

        {

        // Find the number of repeating or non-repeating rows (up to 15)

        int count = 1;

        TBool repeating = EFalse;

        const TUint8* cur = p;

        while (count < 15 && row + count < height)

            {

            const TUint8* prev = cur;

            cur += tt_raster_map.cols;

            TBool same = Mem::Compare(prev,width_bytes,cur,width_bytes) == 0;

            if (count == 1)

                repeating = same;

            else if (repeating != same)

                break;

            count++;

            }

        // Write a 0 for repeating or a 1 for non-repeating.

        WriteGlyphBit(repeating ? 0 : 1);

        // Write the count.

        WriteGlyphBit(count & 1 ? 1 : 0);

        WriteGlyphBit(count & 2 ? 1 : 0);

        WriteGlyphBit(count & 4 ? 1 : 0);

        WriteGlyphBit(count & 8 ? 1 : 0);

        // Write a single repeating row or all the non-repeating rows.

        int rows_written = repeating ? 1 : count;

        cur = p;

        for (int row_written = 0; row_written < rows_written; row_written++, 

       cur += tt_raster_map.cols)

            {

            int col = 0;

            for (int byte = 0; byte < width_bytes; byte++)

                {

                unsigned char x = cur[byte];

                for (int bit = 0; bit < 8 && col < width; bit++, col++, x <<= 1)

                    WriteGlyphBit(x & 128 ? 1 : 0);

                }

            }

        row += count;

        p += tt_raster_map.cols * count;

        }

    EndGlyph();

    if (use_temp_raster_map)

        CleanupStack::PopAndDestroy();    // cur_raster_map

    }</codeblock> </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-5A89E3C4-DEE7-5823-A109-92ED0D87B58C.dita"><linktext>Deriving from