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<concept id="GUID-667E7F90-D6C2-55CE-AE60-6C938072FB9C" xml:lang="en"><title>Graphics
and Drawing Overview</title><shortdesc>This topic provides an introduction to drawing graphics to the
screen. </shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>
<p> <b>Variant</b>: Both (ScreenPlay and non-ScreenPlay). <b>Target
audience</b>: Application developers. </p>
<p>Applications can draw to any <codeph>RDrawableWindow</codeph> —such as
an <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita"><apiname>RWindow</apiname></xref> —via a graphics device, of type <xref href="GUID-30479BE3-296E-3B4D-914D-B080ABD733E4.dita"><apiname>CWsScreenDevice</apiname></xref>,
and a graphics context, of type <xref href="GUID-0AEE5955-C530-35F1-A904-69183331B294.dita"><apiname>CWindowGc</apiname></xref>. These classes
are derived from the <xref href="GUID-B6D4AEE9-5C17-51D9-BBDE-7CCB5218279D.dita">GDI
component</xref> classes <xref href="GUID-500FC564-35E9-3B66-A0C2-1269371A2EA0.dita"><apiname>CGraphicsDevice</apiname></xref> and <xref href="GUID-DAD09DCF-3123-38B4-99E9-91FB24B92138.dita"><apiname>CGraphicsContext</apiname></xref>,
respectively. This means that general drawing functions can be used for drawing
to windows, as well as to other graphics devices. The Window Server itself
does not provide the facilities to draw graphics to a physical device. <codeph>CWindowGc</codeph> functions
are not passed to the Window Server directly. Rather, they are stored in a
buffer maintained by the Window Server Client API. This buffer is flushed
to the Window Server only rarely. By this means the context switching involved
in drawing is minimised, and system performance significantly enhanced. </p>
<p> <codeph>CWsScreenDeviceminimized</codeph> encapsulates the device-dependent
aspects of graphics operations. Graphics functions are not carried out directly
via a <codeph>CWsScreenDevice</codeph>, however, but via a graphics context
with which it is associated. The graphics context class, <codeph>CWindowGc</codeph>,
provides a rich set of drawing functions, including functions to draw lines,
arcs, polygons, text and bitmaps. </p>
<p>A graphics context contains a collection of configurable parameters concerned
with graphics, such as pen width, pen color, brush color. It is stored in
the server, thus reducing the amount of information that has to be sent with
each graphics call. The graphics call simply specifies the graphics context
it wishes to use, and a single graphics context can be shared between multiple
windows. </p>
<p>To draw to a graphics context it must be associated with a window. Typically
a graphics context is created when a session is constructed, and that graphics
context is shared between several windows in the application. When the window
needs to use the graphics context it calls <xref href="GUID-0AEE5955-C530-35F1-A904-69183331B294.dita#GUID-0AEE5955-C530-35F1-A904-69183331B294/GUID-1C0F9DF6-23C2-3707-8F2B-0A738467FF0A"><apiname>CWindowGc::Activate()</apiname></xref>.
If necessary it can change the graphics context's settings. <xref href="GUID-0AEE5955-C530-35F1-A904-69183331B294.dita#GUID-0AEE5955-C530-35F1-A904-69183331B294/GUID-E977DBB2-8547-3AB7-99E3-9D246DA6CF58"><apiname>CWindowGc::Deactivate()</apiname></xref> should
be called first if the graphics context is currently active upon another window. </p>
<p>Several optimizations are used by the Window Server to obtain high-performance
graphics: </p>
<ul>
<li id="GUID-C8AC84D6-AF17-5975-88B8-CC93C499DFA0"><p>Each window is associated
with an <codeph>RWsSession</codeph> which is in turn associated with a client-side
buffer. Instead of implementing graphics operations by a direct client-server
call, which involves expensive context switching, all graphics operations
are stored as opcodes in the buffer, and the buffer is only flushed in certain
circumstances. </p> </li>
<li id="GUID-00B3FC8E-25DE-542F-BF67-0F71CF22526A"><p>The <codeph>CFbsBitmap</codeph> class
allows a bitmap to be shared between all threads in the system, including
the client and the Window Server. This sharing is mediated by the <xref href="GUID-A03FB1BF-F67B-519D-A904-74CA3F8375D9.dita">Font
and Bitmap server</xref>. The <xref href="GUID-17150D76-BB82-3A4B-8B1A-8BA93CB1A9EF.dita"><apiname>CWsBitmap</apiname></xref> class eliminates
further context switches by taking ownership of the handle of the bitmap.
Applications can use this class to more efficiently open, blit-to-screen,
and close a series of bitmaps. Use functions that take a <codeph>CWsBitmap</codeph> in
preference to those that take a <codeph>CFbsBitmap</codeph>, because they
are faster. </p> </li>
<li id="GUID-550D1374-C166-54C6-A055-0CA040691654"><p>A single graphics context
may be used for drawing to many windows—it is not necessary to have one per
window. The <codeph>Activate()</codeph> function associates a <codeph>CWindowGc</codeph> with
a particular window. </p> </li>
<li id="GUID-91AFC884-0BB2-5A73-B757-94EF1B2328A7"><p>Provided drawing operations
to an <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita"><apiname>RWindow</apiname></xref> are performed as <xref href="GUID-8DB1C618-597C-560C-95A2-C0AB2CEBB027.dita">redraw
drawing</xref>, the Window Server stores the sequence of drawing commands
that represent the window contents in redraw stores. Then when the Window
Server needs to repaint the window (because, for example, a dialog box popped
up over it and has now closed) it simply replays the sequence of stored commands,
rather than sending a redraw request to the client. This minimizes the number
of client-server transactions and means that windows are repainted as soon
as the Window Server detects that they are needed. </p> <p>This means that
all <xref href="GUID-0AEE5955-C530-35F1-A904-69183331B294.dita"><apiname>CWindowGc</apiname></xref> drawing should now be redraw drawing, which
means that it takes place between <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita#GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79/GUID-9337538E-7A53-3153-A330-968B5E4F2FF2"><apiname>RWindow::BeginRedraw()</apiname></xref> and <xref href="GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79.dita#GUID-683603DD-F3D3-3193-BEB3-8236C7DE7F79/GUID-3DE16607-AD3B-3946-BEB3-88512EAAB9CE"><apiname>RWindow::EndRedraw()</apiname></xref> calls.
If you use the <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-D5458F8C-E199-37DD-B821-050B749122C6"><apiname>CCoeControl::DrawNow()</apiname></xref> and <xref href="GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160.dita#GUID-B06F99BD-F032-3B87-AB26-5DD6EBE8C160/GUID-9FB682AC-0209-302A-83F3-7BCB1162B998"><apiname>CCoeControl::DrawDeferred()</apiname></xref> methods,
the UI Control Framework (CONE) takes care of this for you. See <xref href="GUID-8DB1C618-597C-560C-95A2-C0AB2CEBB027.dita">Redraw
Drawing</xref> for more information. </p> </li>
</ul>
</conbody><related-links>
<link href="GUID-484B51EC-2209-5492-8E9C-9D792AB0DF35.dita"><linktext>Graphics
and Drawing </linktext></link>
<link href="GUID-6C16417B-5B37-5310-B59A-750D971AA6D4.dita"><linktext>The UI Control
Framework (CONE)</linktext></link>
</related-links></concept>