Qt 5：缩放图像的鼠标滚轮事件行为

Question

我有一个窗口，其中是一个 QGraphicsView，它将显示一个图像。我已经实现了wheelEvent()。我的图像大多会比窗口大，所以我会在窗口中看到滚动条。

当我们在 Windows 照片查看器中查看图像时旋转滚轮时通常观察到的是，当我们向上移动滚轮（朝向它的电线）时，图像会放大，而当我们向下移动它时（朝向身体外），图像缩小。

我得到的是当我将滚轮向自己移动（缩小）图像而不是缩小时，首先向下滚动，并且仅在滚动条触及其最底部时才开始缩小。

最好通过尝试代码来理解问题。我想我无法解释。

我想要标准行为。怎么办？

代码

#include "viewer.h"
#include "ui_viewer.h"
#include <QGraphicsView>
#include <QGraphicsItem>
#include <QGraphicsPixmapItem>
#include <QWheelEvent>
#include <QDebug>
#include <QImage>
#include <QImageReader>
#include <QApplication>
#include <QDesktopWidget>

viewer::viewer(QWidget *parent) : QWidget(parent),ui2(new Ui::viewer)
{
     ui2->setupUi(this);
} 

viewer::~viewer()
{
    delete ui2;
}

int viewer::show_changes(QString folder)
{
    QDesktopWidget *desktop = QApplication::desktop();
    int screenWidth  = desktop->width();
    int screenHeight = desktop->height();
    QString filename = "image_bigger_than_window.jpg";
    QPixmap pixmap = QPixmap(filename);
    QImageReader reader(filename);
    QImage image = reader.read();
    QSize size = image.size();
    int width = 800;
    int height = (width * size.height()) / size.width();
    int x = (screenWidth - width) / 2;
    int y = (screenHeight - height) / 2 - 30;
    setGeometry(x,y,width, height);
    setWindowTitle("OUTPUT");
    ui2->graphicsView->setGeometry(0,0,width,height);
    QGraphicsScene* viewScene = new QGraphicsScene(QRectF(0, 0,width,  height), 0);
    QGraphicsPixmapItem *item = viewScene->addPixmap(pixmap.scaled(QSize((int)viewScene->width(), (int)viewScene->height()),
                             Qt::IgnoreAspectRatio, Qt::SmoothTransformation));
    viewScene->addItem(item);
    ui2->graphicsView->fitInView(QRectF(0, 0, width,     height),Qt::IgnoreAspectRatio);
    ui2->graphicsView->setScene(viewScene);
    ui2->graphicsView->show();
    return 0;
}

void viewer::wheelEvent(QWheelEvent * event)
{
    const int degrees = event->delta() / 8;
    qDebug() << degrees;
    int steps = degrees / 15;        
    double scaleFactor = 1.0; 
    const qreal minFactor = 1.0;
    const qreal maxFactor = 10.0;
    qreal h11 = 1.0, h22 = 0;
    if(steps > 0)
    {
        h11 = (h11 >= maxFactor) ? h11 : (h11 + scaleFactor);
        h22 = (h22 >= maxFactor) ? h22 : (h22 + scaleFactor);
    }
    else
    {
        h11 = (h11 <= minFactor) ? minFactor : (h11 - scaleFactor);
        h22 = (h22 <= minFactor) ? minFactor : (h22 - scaleFactor);
    }
    ui2->graphicsView->setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
    ui2->graphicsView->setTransform(QTransform(h11, 0, 0,0, h22, 0, 0,0,1));

}

编辑

1) 从viewer.cpp 中删除函数void viewer::wheelEvent(QWheelEvent * event)

2) 将bool viewer::eventFilter(QObject *obj, QEvent *event) 作为受保护的函数，将void viewer::handleWheelOnGraphicsScene(QGraphicsSceneWheelEvent* scrollevent) 作为viewer.h 中的公共槽位

bool viewer::eventFilter(QObject *obj, QEvent *event)
{
    if (event->type() == QEvent::GraphicsSceneWheel)
    {    
        QGraphicsSceneWheelEvent *scrollevent = static_cast<QGraphicsSceneWheelEvent *>(event);
        handleWheelOnGraphicsScene(scrollevent);
        return true;
    }
  // Other events should propagate - what do you mean by propagate here?
  return false;
}

 void viewer::handleWheelOnGraphicsScene(QGraphicsSceneWheelEvent* scrollevent)
{
   const int degrees = scrollevent->delta()  / 8;
   qDebug() << degrees;
   int steps = degrees / 15;
   qDebug() << steps;
   double scaleFactor = 1.0; //How fast we zoom
   const qreal minFactor = 1.0;
   const qreal maxFactor = 10.0;
   if(steps > 0)
   {
      h11 = (h11 >= maxFactor) ? h11 : (h11 + scaleFactor);
      h22 = (h22 >= maxFactor) ? h22 : (h22 + scaleFactor);
   }
   else
  {
      h11 = (h11 <= minFactor) ? minFactor : (h11 - scaleFactor);
      h22 = (h22 <= minFactor) ? minFactor : (h22 - scaleFactor);
  } 
ui2->graphicsView->setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
ui2->graphicsView->setTransform(QTransform(h11, 0, 0,0, h22, 0, 0,0,1));

}

Answer 1

代码显示您没有继承QGraphicsView，而是在您自己的小部件中使用了一个。

wheel 事件将首先发送到实际的图形视图小部件。在那里，它使用 Qt 的默认行为进行处理，即滚动。仅当您滚动到底部时，图形视图才能处理滚轮事件，并且它会传播到它的父类，即您的类。这就是为什么您只能在滚动到边框时才能缩放。

要解决此问题，您应该install an event filter。这允许您拦截轮子事件并在您的类中处理它：

// Outline, not tested
viewer::viewer(QWidget *parent) : QWidget(parent),ui2(new Ui::viewer)
{
    ui2->setupUi(this);
    // Let me handle your events
    ui2->graphicsView->installEventFilter(this);
} 

// should be protected
bool viewer::eventFilter(QObject *obj, QEvent *event) {
  if (event->type() == QEvent::GraphicsSceneWheel) {
      // Your implementation. 
      // You can't use QWheelEvent, as Graphicscene works with its own events...
     handleWheelOnGraphicsScene(static_cast<QGraphicsSceneWheelEvent*> (event));

     // Don't propagate
     return true;
  }

  // Other events should propagate
  return false;
}

更新
我刚刚发现事件过滤器不会在图形视图上接收 GraphicsSceneWheel 事件。相反，您必须在图形场景上安装过滤器。此外，您必须调用 event->accept() 以使其不会传播。

所以更新了代码：

// In Constructor, or where appropriate
ui2->graphicsView->scene()->installEventFilter(this);


bool viewer::eventFilter(QObject *obj, QEvent *event) {
    if (event->type() == QEvent::GraphicsSceneWheel) {
        handleWheelOnGraphicsScene(static_cast<QGraphicsSceneWheelEvent*> (event));

        // Don't propagate
        event->accept();
        return true;
    }
    return false;
}

还请注意，handleWheelOnGraphicsScene 或任何您想调用的名称都应该是私有方法，而不必是插槽。