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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>