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Archived:Painting only the dirty area of the widget in Qt

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{{KBCS}}
 
{{CodeSnippet
 
{{CodeSnippet
|id=
+
|id=CS001516
 
|platform=Qt
 
|platform=Qt
|devices=Nokia 5800 eXpressMusic, Nokia N900
+
|devices=Nokia 5800 XpressMusic, Nokia N900
 
|category=Qt
 
|category=Qt
 
|subcategory=UI
 
|subcategory=UI
|creationdate=October 20, 2009
+
|creationdate=November 25, 2009
 
|keywords=WA_OpaquePaintEvent, QPainter::setClipRegion()
 
|keywords=WA_OpaquePaintEvent, QPainter::setClipRegion()
 
}}
 
}}
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==Overview==
 
==Overview==
  
This example shows how to paint only an area of the widget whose content has been marked as invalid and needs to be updated. That will speed up UI painting.
+
This example shows how to paint only the area of the widget whose content has been marked as invalid and needs to be updated. This will speed up UI painting.
  
When user draws line using stylus on the sceen is line first painted into QPixmap picture and
+
When the user draws a line using the stylus on the screen, the line is first painted into a <tt>QPixmap</tt> picture and then the picture is updated to the screen. The application only updates the dirty area of the picture.
then picture is updated into screen. Application updates only dirty area of picture on line drawing.
+
  
Dirty area painting can be handled setting Qt::WA_OpaquePaintEvent attribute of widget as true and
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Dirty area painting can be handled by setting the <tt>Qt::WA_OpaquePaintEvent</tt> attribute of the widget as <tt>true</tt> and defining the dirty rect to <tt>QPainter</tt> with <tt>QPainter::setClipRegion()</tt>.
defining dirty rect to QPainter with QPainter::setClipRegion().
+
  
  
 
==Preconditions==
 
==Preconditions==
  
* Qt is installed on your platform
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* Qt is installed on your platform.
 
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** Download Qt for Symbian release from here: [http://qt.nokia.com/downloads]
 
** Download Qt for Symbian release from here: [http://qt.nokia.com/downloads]
 
 
** Qt on Maemo can be found here: [http://qt4.garage.maemo.org/ Qt4 Maemo port]
 
** Qt on Maemo can be found here: [http://qt4.garage.maemo.org/ Qt4 Maemo port]
  
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===Postconditions===
 
===Postconditions===
  
Screen updates fast.
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The screen updates quickly.
  
  

Revision as of 16:25, 25 November 2009

Template:KBCS

Article Metadata
Tested with
Devices(s): Nokia 5800 XpressMusic, Nokia N900
Compatibility
Platform(s): Qt
Symbian
Article
Keywords: WA_OpaquePaintEvent, QPainter::setClipRegion()
Created: (25 Nov 2009)
Last edited: copyeditor (25 Nov 2009)

Contents

Overview

This example shows how to paint only the area of the widget whose content has been marked as invalid and needs to be updated. This will speed up UI painting.

When the user draws a line using the stylus on the screen, the line is first painted into a QPixmap picture and then the picture is updated to the screen. The application only updates the dirty area of the picture.

Dirty area painting can be handled by setting the Qt::WA_OpaquePaintEvent attribute of the widget as true and defining the dirty rect to QPainter with QPainter::setClipRegion().


Preconditions

  • Qt is installed on your platform.
    • Download Qt for Symbian release from here: [1]
    • Qt on Maemo can be found here: Qt4 Maemo port


Header

#include <QtGui>
 
class Widget: public QWidget
{
Q_OBJECT
 
public:
Widget(QWidget *parent = 0);
~Widget();
 
void mouseMoveEvent(QMouseEvent *);
void mousePressEvent(QMouseEvent *);
void mouseReleaseEvent(QMouseEvent *);
 
void paintEvent(QPaintEvent *);
void resizeEvent(QResizeEvent *);
 
private:
void paintToPixmap();
void paintLine(QPainter&);
QRect dirtyRect();
 
private:
QPoint p1;
QPoint p2;
 
QPen pen;
QColor color;
 
QPixmap pixmap;
};


Source

Widget::Widget(QWidget *parent) :
QWidget(parent)
{
p1 = QPoint(-1, -1);
p2 = QPoint(-1, -1);
 
color = QColor(Qt::black);
pen.setColor(color);
pen.setStyle(Qt::SolidLine);
pen.setWidth(2);
}
 
Widget::~Widget()
{
}
 
void Widget::resizeEvent(QResizeEvent *event)
{
QWidget::resizeEvent(event);
// Create pixmap that size is same as screen has
pixmap = QPixmap(event->size());
pixmap.fill(Qt::white);
}
 
void Widget::mousePressEvent(QMouseEvent *event)
{
p1 = event->pos();
p2 = QPoint(-1, -1);
 
// Opaquea paint, we paint only changed area of the screen
setAttribute(Qt::WA_OpaquePaintEvent, true);
}
 
void Widget::mouseMoveEvent(QMouseEvent *event)
{
if (p2.x() == -1) {
p2 = event->pos();
}
else {
p1 = p2;
p2 = event->pos();
}
// Paint to pixmap and then pixmap on screen
paintToPixmap();
}
 
void Widget::mouseReleaseEvent(QMouseEvent *)
{
// Stop using opaquea paint
setAttribute(Qt::WA_OpaquePaintEvent, false);
 
// Update screen
update();
}
 
void Widget::paintToPixmap()
{
// Paint line to pixmap
QPainter painter;
painter.begin(&pixmap);
painter.setRenderHint(QPainter::Antialiasing);
painter.setPen(pen);
paintLine(painter);
painter.end();
// Draw pixmap on the screen
update();
}
 
void Widget::paintLine(QPainter& painter)
{
if (p1.x() != -1 && p2.x() != -1) {
painter.drawLine(p1, p2);
}
}
 
QRect Widget::dirtyRect()
{
int left, top, width, height;
if (p1.x() == -1 || p2.x() == -1) {
return rect();
}
 
if (p1.x() < p2.x()) {
left = p1.x();
width = p2.x() - p1.x();
}
else {
left = p2.x();
width = p1.x() - p2.x();
}
 
if (p1.y() < p2.y()) {
top = p1.y();
height = p2.y() - p1.y();
}
else {
top = p2.y();
height = p1.y() - p2.y();
}
return QRect(left - 15, top - 15, width + 30, height + 30);
}
 
void Widget::paintEvent(QPaintEvent *)
{
QPainter painter(this);
 
// Paint only changed area of the screen
if (testAttribute(Qt::WA_OpaquePaintEvent)) {
painter.setClipRegion(dirtyRect());
}
 
// Update pixmap into screen. If dirty
// area is set painter updates only that
painter.drawPixmap(QPoint(0, 0), pixmap);
}


Postconditions

The screen updates quickly.


See also

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