【C++】Qt组件QScrollArea的图上选点功能实现

一、基本信息

编程语言：C++
可视化平台：Qt
主要组件：QScrollArea，QLable

二、要点

1.QScrollArea

（1）用QScrollArea添加图片的代码如下：

QScrollArea scrollArea;
QLabel imageLabel;
QPixmap pixMap;
imageLabel.setPixmap(pixMap);
scrollArea.setWidget(imageLable);

（2）滚动区坐标与实际图片中坐标的转换

坐标转换公式为 QImage(X,Y)=QScroll Value(h,v)+QReal(x,y)

PImage(X,Y) 为图像的实际坐标， PScroll Value(h,v) 为水平、垂直滚动条的value()值构成的坐标， PReal(x,y) 为scrollArea的相对坐标。

这里有一个简单的例子可以解释ScrollBar的value()值和步长pageStep()之间的关系（摘自Qt Helper）：
If we have a document with 100 lines, and we can only show 20 lines in a widget, we may wish to construct a scroll bar with a page step of 20, a minimum value of 0, and a maximum value of 80. This would give us a scroll bar with five “pages”.

获取QLable的相对坐标的方法可参考云鹤起舞的Qt获取鼠标位置（绝对位置、相对位置）一文

QPoint *qrealPoint;
qrealPoint= event->globalPos(); //获取全局位置，event为鼠标单击事件
qrealPoint= ui->leftImgLabel>mapFromGlobal(qrealPoint);

获取滚动条value()值的方法如下：

QScrollBar *scrollBar_X = scrollArea->horizontalScrollBar();
float x_offset = scrollBar_X->value();

2.Qt事件过滤器

此处用于实现不同QLable的单击选点事件处理，事件过滤器的主要使用方法如下：

.h文件中声明bool eventFilter(QObject *obj, QEvent *eve);
.cpp构造函数中注册ui.sa_geoImage->installEventFilter(this);
.cpp中重写eventFilter()方法

三、代码

/**
 *  @BRIEF Reimplement eventFilter()
 *  @param *obj: Which object we should control
 *  @param *eve: Which event has happened
 */
bool MatchWindowSingle::eventFilter(QObject *obj, QEvent *eve)
{
    if (obj == ui.sa_geoImage){                             //Check whether it's a geoImage Scroll Area
        if (selectPolicy == GEOIMAGE_SELECTING){            //Check whether geoImage can be selected
            if (eve->type() == QEvent::MouseButtonRelease){ //Check mouse event
                QMouseEvent *mouseEvent = static_cast(eve);
                if (mouseEvent->button() == Qt::LeftButton){ // Left button click

                    //Get Image Position of the Select Point.
                    Point2f imagePoint = getImageCoordinate(mouseEvent, ui.sa_geoImage, &scaleFactor_g);                

                    //Draw point in the screen.     
                    int pointSize = (int)(2/scaleFactor_MIN_g);
                    int pix_x = (int)imagePoint.x;
                    int pix_y = (int)imagePoint.y;              
                    QPen pen(QColor(250,112,154));              //Set pen
                    QBrush brush(QColor(250,112,154));  //Set brush
                    QPainter painter(&geoPic);      
                    painter.setPen(pen);
                    painter.setBrush(brush);
                    painter.drawEllipse(QPoint(pix_x,pix_y), pointSize, pointSize);
                    painter.setRenderHint(QPainter::Antialiasing);//Make it more smooth 

                    //Update the scrollArea.
                    updateScrollArea(&geoPic, ui.sa_geoImage, &geoImageLable, &scaleFactor_g);
                    selectPolicy = VIDEOIMAGE_SELECTING;
                    return true;
                }else{return false;}
            }else{return false;}
        }else{return false;}        
    }else if (obj == ui.sa_videoImage){                     //Check whether it's a videoImage Scroll Area
        if (selectPolicy == VIDEOIMAGE_SELECTING){          //Check whether videoImage can be selected
            if (eve->type() == QEvent::MouseButtonRelease){ //Check mouse event
                QMouseEvent *mouseEvent = static_cast(eve);
                if (mouseEvent->button() == Qt::LeftButton){ // Left button click

                    //Get Image Position of the Select Point.
                    Point2f imagePoint = getImageCoordinate(mouseEvent, ui.sa_videoImage, &scaleFactor_v);              

                    //Draw point in the screen.     
                    int pointSize = (int)(2/scaleFactor_MIN_v);
                    int pix_x = (int)imagePoint.x;
                    int pix_y = (int)imagePoint.y;              
                    QPen pen(QColor(254,225,64));               //Set pen
                    QBrush brush(QColor(254,225,64));   //Set brush
                    QPainter painter(&videoPic);        
                    painter.setPen(pen);
                    painter.setBrush(brush);
                    painter.drawEllipse(QPoint(pix_x,pix_y), pointSize, pointSize);
                    painter.setRenderHint(QPainter::Antialiasing);//Make it more smooth 

                    //Update the scrollArea.
                    updateScrollArea(&videoPic, ui.sa_videoImage, &videoImageLable, &scaleFactor_v);
                    selectPolicy = GEOIMAGE_SELECTING;
                    return true;
                }else{return false;}
            }else{return false;}
        }else{return false;}
    }else{return false;}
}