Редактирование: Qtopia Core 4.2 Screen Drivers

[[Категория:Qt_Издания]]
__NOTOC__

by Hеvard Wall
<div class="introduction">
QScreen is a class that is specific to Qtopia Core (previously calledQt/Embedded). It is the base class for Qtopia Core's built-in andcustom screen drivers. In this article, we will review the changesthat were made to QScreen in Qtopia Core 4.2 and show how to takefull advantage of hardware acceleration.

*[[#multiheadsupport | Multihead Support]]
*[[#windowsurfaceabstraction | Window Surface Abstraction]]
*[[#writinganacceleratedpaintengine | Writing an Accelerated Paint Engine]]
</div><div id="multiheadsupport"></div>
===Multihead Support===

[[Qt:Документация_4.3.2/qscreen  | QScreen]] now features two virtual functions that can bereimplemented by subclasses to support multiple screens:
<source lang="cpp-qt">
QList<QScreen *> subScreens() const;
QRegion region() const;
</source> 
Qtopia Core 4.2 also include a "Multi" screen driver, which createsother [[Qt:Документация_4.3.2/qscreen  | QScreen]] instances and forwards all function calls to theappropriate [[Qt:Документация_4.3.2/qscreen  | QScreen]]. Here's an example of how to run a QtopiaCore application on three different screens simultaneously:<source lang="cpp-qt">
app -qws -display "Multi: QVFb:0 QVFb:offset=640,0:1 VNC"
</source> Using <tt>-display</tt>, we tell Qtopia Core to create two instances ofthe Qt virtual buffer driver ("[[Qt:Документация_4.3.2/qtopiacore-qvfb  | QVFb]]") and one instance of the VNCdriver. The second QVFb instance is given an offset of (640, 0), sothat it doesn't overlap the first instance.

The coordinates passed to your reimplementations of[[Qt:Документация_4.3.2/qscreen  | QScreen]]'s virtual functions are always specifiedin global coordinates. In most cases, your existing custom drivers will workwith no change, but if you perform some kind of lookup in a data structurethat assumes the screen's origin is (0, 0), you will need to subtract[http://www.crossplatform.ru/documentation/qtdoc4.3/qscreen.php#offset QScreen::offset()] to convert the globalcoordinates into screen coordinates.
<div id="windowsurfaceabstraction"></div>
===Window Surface Abstraction===

Qtopia Core 4.2 introduces "window surfaces" as a mechanism to control the memory used when painting a [[Qt:Документация_4.3.2/qwidget  | QWidget]].This provides a way to accelerate painting beyond what is possible byreimplementing [[Qt:Документация_4.3.2/qscreen  | QScreen]]'s <tt>solidFill()</tt> and<tt>blit()</tt> functions. To support window surfaces, these factory functionswere added to [[Qt:Документация_4.3.2/qscreen  | QScreen]]:
<source lang="cpp-qt">
QWSWindowSurface *createSurface(QWidget *widget) const;
QWSWindowSurface *createSurface(const QString &amp;key) const;
</source> 
[[Qt:Документация_4.3.2/qwswindowsurface  | QWSWindowSurface]] is the abstraction of thearea a top-level [[Qt:Документация_4.3.2/qwidget  | QWidget]] (i.e., a window) is drawnonto. By default, if you are creating a window in the QWS server, this will bea chunk of local memory. If you are creating a window in a client process, itwill be a chunk of shared memory. And if you set the<tt>Qt::WA_PaintOnScreen</tt> attribute on the window, it will be an areainside the framebuffer. This default behavior can be overridden byreimplementing the <tt>createSurface()</tt> functions in a subclass.

If you're writing a driver for a device with little memory and noframebuffer, you probably want to avoid drawing into memory at alland rather use an I/O controller to send drawing commands directlyto the video card. To achieve this, you would subclass[[Qt:Документация_4.3.2/qwswindowsurface  | QWSWindowSurface]] to avoid the memoryallocation and reimplement the <tt>paintDevice()</tt> function to return a[[Qt:Документация_4.3.2/qcustomrasterpaintdevice  | QCustomRasterPaintDevice]] subclass.The paint device's <tt>paintEngine()</tt> function would return an instance ofan accelerated paint engine.
<div id="writinganacceleratedpaintengine"></div>
===Writing an Accelerated Paint Engine===

Writing an accelerated paint engine consists of subclassing[[Qt:Документация_4.3.2/qrasterpaintengine  | QRasterPaintEngine]] and reimplementingthe following two virtual functions, which are new in Qtopia Core 4.2:
<source lang="cpp-qt">
void drawColorSpans(const QSpan *spans, int count,
                    uint color);
void drawBufferSpan(const uint *buffer, int bufferSize,
                    int x, int y, int length,
                    uint constantAlpha);
</source> 
These functions are only called if the paint device's <tt>memory()</tt>function is reimplemented to return 0. The <tt>drawColorSpans()</tt>function is called to draw one or more spans with the given coloron the screen. The <tt>drawBufferSpan()</tt> function is called to draw abuffer of pixels from memory.

Be aware that both [http://doc.trolltech.com/4.2/qwswindowsurface.html QWSWindowSurface] andthe accelerated paint engine technique described above are included in QtopiaCore 4.2 as "preliminary APIs", meaning that they may change in a laterrelease.
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