How to transffer QImage or QPixmap from one window to another using signals and slots

JonB

Either you do not hit that [emit] code or you have not connected the correct signal instance to the correct slot instance

swansorter

@JonB i tried qDebug() << "EMIT" near the emit signal. and it is executing

JonB

@swansorter
Then

or you have not connected the correct signal instance to the correct slot instance

(or, for example, your signal or slot objects have gone out of scope or life-time. I don't know what else to say. You know that signals & slots work, so there is a problem in your code.

Produce a minimal example. To rule out a problem with transferring a const QImage & as the parameter, temporarily change signal & slot to transfer, say, a QString, and emit sendImage("Hello"); till you get that working....

swansorter

@JonB i changed to Qstring and hello is not displaying. how to rectify this problem

jsulm

@swansorter said in How to transffer QImage or QPixmap from one window to another using signals and slots:

my_qlabel *test_label=new my_qlabel();
connect(test_label,&my_qlabel::sendImage,this,&MainWindow::onGetImage);

Is it possible that you're creating another test_label somewhere and emit the signal there?
Without more information (code) we can't treally help.

KroMignon

@swansorter said in How to transffer QImage or QPixmap from one window to another using signals and slots:

i changed to Qstring and hello is not displaying. how to rectify this problem

Are you sure the connect() statement was successful?
Are you sure to use the right class instance?
Can you show my_qlabel class header?

JonB

@swansorter said in How to transffer QImage or QPixmap from one window to another using signals and slots:

how to rectify this problem

Like I said, produce a minimal example, paste the code, and we will tell you what you are doing wrong.....

swansorter

@JonB

mainwindow.h file
#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include<QRubberBand>
#include<my_qlabel.h>
namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
       static QPoint origin;
       static QPoint Move_test;
       static QRect f_r;
 private slots:
    void onGetImage (QString data);



private:
    Ui::MainWindow *ui;
    my_qlabel *obj;
    QPoint origin_x;

};

#endif // MAINWINDOW_H

``
my qlable.h file`
#ifndef MY_QLABEL_H
#define MY_QLABEL_H

#include <QWidget>
#include <QLabel>
#include <QEvent>
#include <QMouseEvent>
#include <QDebug>
#include<QRubberBand>
class my_qlabel : public QLabel
{
Q_OBJECT
public:
explicit my_qlabel(QWidget *parent = 0);
void mouseMoveEvent(QMouseEvent *ev);
void mousePressEvent(QMouseEvent *ev);
void mouseReleaseEvent(QMouseEvent *ev);
int x,y;
signals:
void onSendImage(QString data);

public slots:
private:

QRubberBand *rubberBand=0;

QPoint origin_x;

};

#endif // MY_QLABEL_H

#include "mainwindow.h"
#include "ui_mainwindow.h"
QPoint MainWindow::origin;
QPoint MainWindow::Move_test;
QRect MainWindow::f_r;
MainWindow::MainWindow(QWidget *parent) :
QMainWindow(parent),
ui(new Ui::MainWindow)
{
ui->setupUi(this);
ui->label->setStyleSheet("QLabel{background: yellow;}");
obj=new my_qlabel(this);
connect(obj,SIGNAL(onSendimage(QString)),this,SLOT(onGetimage(QString)));
}

MainWindow::~MainWindow()
{
delete ui;
}

void MainWindow::onGetimage (QString data)
{
QPixmap image=data ;
ui->label_2->setPixmap(image);
}

#include "my_qlabel.h"
#include<mainwindow.h>
my_qlabel::my_qlabel(QWidget *parent) :
QLabel(parent)
{
emit Mouse_Left("image");
}

void my_qlabel::mouseMoveEvent(QMouseEvent *ev)
{
this->x = ev->x();
this->y = ev->y();
rubberBand->setGeometry(QRect(origin_x,ev->pos()).normalized());

}

void my_qlabel::mousePressEvent(QMouseEvent *ev)
{

origin_x=ev->pos();
if(!rubberBand)
    rubberBand=new QRubberBand(QRubberBand::Rectangle,this);
rubberBand->setGeometry(QRect((origin_x),QSize()));
rubberBand->show();

}

void my_qlabel::mouseReleaseEvent(QMouseEvent *ev)
{

    rubberBand->hide();

    MainWindow::f_r=rubberBand->geometry();
    qDebug()<<"f_r"<<MainWindow::f_r;
    QPixmap  image(rubberBand->geometry().width(),rubberBand->geometry().height());
    image = grab(rubberBand->geometry()); //copy the selected part

    emit onSendimage("image");

}

swansorter

@KroMignon yes i am using right class instance . i have pasted my code

KroMignon

@swansorter said in How to transffer QImage or QPixmap from one window to another using signals and slots:

yes i am using right class instance . i have pasted my code

Your code seem at little bit confusing to me.
Why do you use old connect syntax here:

connect(obj,SIGNAL(Mouse_Left(QString)),this,SLOT(Mouse_Left(QString)));

and not new syntax?

connect(obj,&my_qlabel::Mouse_Left,this, &MainWindow::Mouse_Left);

Where did you add obj to the layout?

jsulm

@swansorter And please be so kind and format your code properly to make it easier for others to read it...

swansorter

@JonB i resolved issue .by changing this
connect(obj,SIGNAL(Mouse_Left(QString)),this,SLOT(Mouse_Left(QString)));
to
connect(ui.label,SIGNAL(Mouse_Left(QString)),this,SLOT(Mouse_Left(QString)));
}

JonB

@swansorter
This is hardly the minimal example I asked for....

Do not use SIGNAL/SLOT macros if you are to have any chance of debugging what is going on. You do not even check your connect()s return result. Use the new syntax only.

You declare Mouse_Left as a signal, yet you provide your own implementation of a method with that name. I don't know that you can do that.

I'm quite mixed up about what are signals and what are slots in main window vs the label.

If you want me to look further: Name your slots differently from your signals, else it's too confusing. Reduce to a minimal example. And enclose blocks of code with the form posts' Code tag.

UPDATE
Crossed with your latest post. Advice: get rid of SIGNAL/SLOT in all your code, only use new style, to help both you and others.

swansorter

@jsulm sorry ill do that

swansorter

@JonB do you have the link for the new style SIGNAL/SLOT Diclaration

jsulm

@swansorter said in How to transffer QImage or QPixmap from one window to another using signals and slots:

do you have the link for the new style SIGNAL/SLOT Diclaration

Easy to find with Google: https://doc.qt.io/qt-5/signalsandslots.html

JonB

@swansorter
Additionally to @jsulm, you originally had it with your

connect(test_label,&my_qlabel::sendImage,this,&MainWindow::onGetImage);

You then changed over to SIGNAL/SLOT, for no discernible reason, which may be a clue as to why it went wrong....