Show grey level value of pixel in Qt when the mouse is over it - image

Question

Currently I am working on the display of gray level image with zoom feature. I am able to get the position of the pixel and the zoom feature is working well. However I encountered two problems:
1.) How can I get the grey level value of the pixel that is pointed by the mouse? I only managed to obtain the rgb value through “QRgb rgbValue = pix.toImage().pixel(x,y)”. How can I convert it into grey level value? Or is there any direct way to get the grey level value of the pixel.
2.) I have implemented “mouseMoveEvent(QMouseEvent *event)” and “setMouseTracking(true)”. However the function of “mouseMoveEvent(QMouseEvent *event)” is not functioning when I move the mouse. What is wrong with my code?
mainwindow.h
#ifndef MAINWINDOW_H
#define MAINWINDOW_H
#include <QMainWindow>
#include <QGraphicsScene>
#include <QGraphicsItem>
namespace Ui {
class MainWindow;
}
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
private slots:
void on_pushButton_clicked();
protected:
void mouseMoveEvent(QMouseEvent * event);
private:
Ui::MainWindow *ui;
QGraphicsScene* scene;
QGraphicsItem* item;
QPixmap pix;
};
#endif // MAINWINDOW_H
mainwindow.cpp
#include "mainwindow.h"
#include "ui_mainwindow.h"
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
QImage image("E:/image_00002.bmp");
pix = QPixmap::fromImage(image);
scene = new QGraphicsScene(this);
ui->graphicsView->setScene(scene);
scene->addPixmap(pix);
}
MainWindow::~MainWindow()
{
delete ui;
}
void MainWindow::on_pushButton_clicked()
{
ui->graphicsView->setMouseTracking(true);
}
void MainWindow::mouseMoveEvent(QMouseEvent *event)
{
QPoint local_pt = ui->graphicsView->mapFromGlobal(event->globalPos());
QPointF img_coord_pt = ui->graphicsView->mapToScene(local_pt);
double x = img_coord_pt.x();
double y = img_coord_pt.y();
/* How can I get a gray level image here */
QRgb rgbValue = pix.toImage().pixel(x,y);
ui->label_X->setText(QString::number(x));
ui->label_Y->setText(QString::number(y));
ui->label_Value->setText(QString::number(rgbValue));
}

Answer 1

To elaborate a bit on what hank said. In order for your QMainWindow to receive the events from the QGraphicsScene you need to install an event filter (see http://qt-project.org/doc/qt-4.8/qobject.html#installEventFilter ). Quoting from the DOCs:
An event filter is an object that receives all events that are sent to this object.
The filter can either stop the event or forward it to this object.
In order to process the events you need to define an eventFilter method in your main window class. Below are the on_pushButton_clicked() and eventFilter() methods that should do what you want:
void MainWindow::on_pushButton_clicked()
{
ui->graphicsView->setMouseTracking(true);
scene->installEventFilter(this);
}
bool MainWindow::eventFilter(QObject *obj, QEvent *event)
{
if (event->type() == QEvent::GraphicsSceneMouseMove ) {
QGraphicsSceneMouseEvent *mouseEvent = static_cast<QGraphicsSceneMouseEvent*>(event);
QPointF img_coord_pt = mouseEvent->scenePos();
double x = img_coord_pt.x();
double y = img_coord_pt.y();
QColor color = QColor(pix.toImage().pixel(x,y));
int average = (color.red()+color.green()+color.blue())/3;
ui->label_X->setText(QString::number(x));
ui->label_Y->setText(QString::number(y));
ui->label_Value->setText(QString::number(average));
return true;
} else {
return QObject::eventFilter(obj, event);
}
}
Does this help?