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# mandelbrotwidget.cpp Example File
threads/mandelbrot/mandelbrotwidget.cpp

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     #include <QtGui>

     #include <math.h>

     #include "mandelbrotwidget.h"

     const double DefaultCenterX = -0.637011f;
     const double DefaultCenterY = -0.0395159f;
     const double DefaultScale = 0.00403897f;

     const double ZoomInFactor = 0.8f;
     const double ZoomOutFactor = 1 / ZoomInFactor;
     const int ScrollStep = 20;

     MandelbrotWidget::MandelbrotWidget(QWidget *parent)
         : QWidget(parent)
     {
         centerX = DefaultCenterX;
         centerY = DefaultCenterY;
         pixmapScale = DefaultScale;
         curScale = DefaultScale;

         qRegisterMetaType<QImage>("QImage");
         connect(&thread, SIGNAL(renderedImage(const QImage &, double)),
                 this, SLOT(updatePixmap(const QImage &, double)));

         setWindowTitle(tr("Mandelbrot"));
     #ifndef QT_NO_CURSOR
         setCursor(Qt::CrossCursor);
     #endif
         resize(550, 400);
     }

     void MandelbrotWidget::paintEvent(QPaintEvent * /* event */)
     {
         QPainter painter(this);
         painter.fillRect(rect(), Qt::black);

         if (pixmap.isNull()) {
             painter.setPen(Qt::white);
             painter.drawText(rect(), Qt::AlignCenter,
                              tr("Rendering initial image, please wait..."));
             return;
         }

         if (curScale == pixmapScale) {
             painter.drawPixmap(pixmapOffset, pixmap);
         } else {
             double scaleFactor = pixmapScale / curScale;
             int newWidth = int(pixmap.width() * scaleFactor);
             int newHeight = int(pixmap.height() * scaleFactor);
             int newX = pixmapOffset.x() + (pixmap.width() - newWidth) / 2;
             int newY = pixmapOffset.y() + (pixmap.height() - newHeight) / 2;

             painter.save();
             painter.translate(newX, newY);
             painter.scale(scaleFactor, scaleFactor);
             QRectF exposed = painter.matrix().inverted().mapRect(rect()).adjusted(-1, -1, 1, 1);
             painter.drawPixmap(exposed, pixmap, exposed);
             painter.restore();
         }

         QString text = tr("Use mouse wheel or the '+' and '-' keys to zoom. "
                           "Press and hold left mouse button to scroll.");
         QFontMetrics metrics = painter.fontMetrics();
         int textWidth = metrics.width(text);

         painter.setPen(Qt::NoPen);
         painter.setBrush(QColor(0, 0, 0, 127));
         painter.drawRect((width() - textWidth) / 2 - 5, 0, textWidth + 10,
                          metrics.lineSpacing() + 5);
         painter.setPen(Qt::white);
         painter.drawText((width() - textWidth) / 2,
                          metrics.leading() + metrics.ascent(), text);
     }

     void MandelbrotWidget::resizeEvent(QResizeEvent * /* event */)
     {
         thread.render(centerX, centerY, curScale, size());
     }

     void MandelbrotWidget::keyPressEvent(QKeyEvent *event)
     {
         switch (event->key()) {
         case Qt::Key_Plus:
             zoom(ZoomInFactor);
             break;
         case Qt::Key_Minus:
             zoom(ZoomOutFactor);
             break;
         case Qt::Key_Left:
             scroll(-ScrollStep, 0);
             break;
         case Qt::Key_Right:
             scroll(+ScrollStep, 0);
             break;
         case Qt::Key_Down:
             scroll(0, -ScrollStep);
             break;
         case Qt::Key_Up:
             scroll(0, +ScrollStep);
             break;
         default:
             QWidget::keyPressEvent(event);
         }
     }

     void MandelbrotWidget::wheelEvent(QWheelEvent *event)
     {
         int numDegrees = event->delta() / 8;
         double numSteps = numDegrees / 15.0f;
         zoom(pow(ZoomInFactor, numSteps));
     }

     void MandelbrotWidget::mousePressEvent(QMouseEvent *event)
     {
         if (event->button() == Qt::LeftButton)
             lastDragPos = event->pos();
     }

     void MandelbrotWidget::mouseMoveEvent(QMouseEvent *event)
     {
         if (event->buttons() & Qt::LeftButton) {
             pixmapOffset += event->pos() - lastDragPos;
             lastDragPos = event->pos();
             update();
         }
     }

     void MandelbrotWidget::mouseReleaseEvent(QMouseEvent *event)
     {
         if (event->button() == Qt::LeftButton) {
             pixmapOffset += event->pos() - lastDragPos;
             lastDragPos = QPoint();

             int deltaX = (width() - pixmap.width()) / 2 - pixmapOffset.x();
             int deltaY = (height() - pixmap.height()) / 2 - pixmapOffset.y();
             scroll(deltaX, deltaY);
         }
     }

     void MandelbrotWidget::updatePixmap(const QImage &image, double scaleFactor)
     {
         if (!lastDragPos.isNull())
             return;

         pixmap = QPixmap::fromImage(image);
         pixmapOffset = QPoint();
         lastDragPos = QPoint();
         pixmapScale = scaleFactor;
         update();
     }

     void MandelbrotWidget::zoom(double zoomFactor)
     {
         curScale *= zoomFactor;
         update();
         thread.render(centerX, centerY, curScale, size());
     }

     void MandelbrotWidget::scroll(int deltaX, int deltaY)
     {
         centerX += deltaX * curScale;
         centerY += deltaY * curScale;
         update();
         thread.render(centerX, centerY, curScale, size());
     }