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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"));
            setCursor(Qt::CrossCursor);
            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);
                painter.drawPixmap(0, 0, pixmap);
                painter.restore();
            }

            QString text = tr("Use mouse wheel 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());
        }