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

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==Header==
 
==Header==
  
<code cpp>
+
<code cpp-qt>
 
#include <QtGui>
 
#include <QtGui>
  
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==Source==
 
==Source==
  
<code cpp>
+
<code cpp-qt>
 
Widget::Widget(QWidget *parent) :
 
Widget::Widget(QWidget *parent) :
 
     QWidget(parent)
 
     QWidget(parent)
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* [http://doc.trolltech.com/4.5/qpainter.html QPainter]
 
* [http://doc.trolltech.com/4.5/qpainter.html QPainter]
  
[[Category:Qt C++ UI]][[Category:Code Snippet]][[Category: UI]][[Category:Code Snippet]][[Category:MeeGo]] [[Category:Symbian]]
+
[[Category:Qt C++ UI]][[Category:Code Snippet]][[Category: UI]][[Category:Code Snippet]][[Category:MeeGo Harmattan]] [[Category:Symbian]]

Latest revision as of 04:14, 11 October 2012

Archived.pngArchived: This article is archived because it is not considered relevant for third-party developers creating commercial solutions today. If you think this article is still relevant, let us know by adding the template {{ReviewForRemovalFromArchive|user=~~~~|write your reason here}}.

Qt Quick should be used for all UI development on mobile devices. The approach described in this article (using C++ for the Qt app UI) is deprecated.

Article Metadata
Tested with
Devices(s): Nokia 5800 XpressMusic, Nokia N900
Compatibility
Platform(s): Qt
Symbian
Article
Keywords: WA_OpaquePaintEvent, QPainter::setClipRegion()
Created: tepaa (20 Oct 2009)
Last edited: hamishwillee (11 Oct 2012)

Contents

[edit] 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 the updating of the screen.

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().


[edit] Preconditions

  • Qt is installed on your platform.


[edit] 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;
};


[edit] 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);
}


[edit] Postconditions

The screen updates quickly.


[edit] See also

This page was last modified on 11 October 2012, at 04:14.
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