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paintarea.cpp Example File
tools/plugandpaint/paintarea.cpp

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 #include "interfaces.h"
 #include "paintarea.h"

 #include <QPainter>
 #include <QMouseEvent>

 PaintArea::PaintArea(QWidget *parent) :
     QWidget(parent),
     theImage(500, 400, QImage::Format_RGB32),
     color(Qt::blue),
     thickness(3),
     brushInterface(0),
     lastPos(-1, -1)
 {
     setAttribute(Qt::WA_StaticContents);
     setAttribute(Qt::WA_NoBackground);

     theImage.fill(qRgb(255, 255, 255));
 }

 bool PaintArea::openImage(const QString &fileName)
 {
     QImage image;
     if (!image.load(fileName))
         return false;

     setImage(image);
     return true;
 }

 bool PaintArea::saveImage(const QString &fileName, const char *fileFormat)
 {
     return theImage.save(fileName, fileFormat);
 }

 void PaintArea::setImage(const QImage &image)
 {
     theImage = image.convertToFormat(QImage::Format_RGB32);
     update();
     updateGeometry();
 }

 void PaintArea::insertShape(const QPainterPath &path)
 {
     pendingPath = path;
     setCursor(Qt::CrossCursor);
 }

 void PaintArea::setBrushColor(const QColor &color)
 {
     this->color = color;
 }

 void PaintArea::setBrushWidth(int width)
 {
     thickness = width;
 }

 void PaintArea::setBrush(BrushInterface *brushInterface, const QString &brush)
 {
     this->brushInterface = brushInterface;
     this->brush = brush;
 }

 QSize PaintArea::sizeHint() const
 {
     return theImage.size();
 }

 void PaintArea::paintEvent(QPaintEvent * /* event */)
 {
     QPainter painter(this);
     painter.drawImage(QPoint(0, 0), theImage);
 }

 void PaintArea::mousePressEvent(QMouseEvent *event)
 {
     if (event->button() == Qt::LeftButton) {
         if (!pendingPath.isEmpty()) {
             QPainter painter(&theImage);
             setupPainter(painter);

             const QRectF boundingRect = pendingPath.boundingRect();
             QLinearGradient gradient(boundingRect.topRight(),
                                      boundingRect.bottomLeft());
             gradient.setColorAt(0.0, QColor(color.red(), color.green(),
                                             color.blue(), 63));
             gradient.setColorAt(1.0, QColor(color.red(), color.green(),
                                             color.blue(), 191));
             painter.setBrush(gradient);
             painter.translate(event->pos() - boundingRect.center());
             painter.drawPath(pendingPath);

             pendingPath = QPainterPath();
             unsetCursor();
             update();
         } else {
             if (brushInterface) {
                 QPainter painter(&theImage);
                 setupPainter(painter);
                 const QRect rect = brushInterface->mousePress(brush, painter,
                                                               event->pos());
                 update(rect);
             }

             lastPos = event->pos();
         }
     }
 }

 void PaintArea::mouseMoveEvent(QMouseEvent *event)
 {
     if ((event->buttons() & Qt::LeftButton) && lastPos != QPoint(-1, -1)) {
         if (brushInterface) {
             QPainter painter(&theImage);
             setupPainter(painter);
             const QRect rect = brushInterface->mouseMove(brush, painter, lastPos,
                                                          event->pos());
             update(rect);
         }

         lastPos = event->pos();
     }
 }

 void PaintArea::mouseReleaseEvent(QMouseEvent *event)
 {
     if (event->button() == Qt::LeftButton && lastPos != QPoint(-1, -1)) {
         if (brushInterface) {
             QPainter painter(&theImage);
             setupPainter(painter);
             QRect rect = brushInterface->mouseRelease(brush, painter,
                                                       event->pos());
             update(rect);
         }

         lastPos = QPoint(-1, -1);
     }
 }

 void PaintArea::setupPainter(QPainter &painter)
 {
     painter.setRenderHint(QPainter::Antialiasing, true);
     painter.setPen(QPen(color, thickness, Qt::SolidLine, Qt::RoundCap,
                    Qt::RoundJoin));
 }


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Qt 4.3.5