QPixmap cropping using a QRect

FED27

Hi to everyone.
It's my first post and i've installed qt with opencv libraries recently. I' m trying to crop a part of an area of a video displayed in a QLabel. i've subclassed this QLabel in order to have mousePressEvent and other function.

When it crops the area enclosed by the rectangle it's not exactly the area contained in this.

When i chose the point of rect i used

selectionRect.setTopLeft(e->pos()) and selectionRect.setBottomRight(e->pos());
Are these points the problem?

Thanks

mrjj

Hi

it's not exactly the area contained in this.

How much is it off?

I assume e->pos() is from MousePress event and should already converted to client coordinates?

You should qDebug() << selectionRect; and see what you get of values.

It seems a bit wrong as it seems that u make a rect with the same 2 points which
im not sure is correct or just seems so as rest of code is not there.

selectionRect.setTopLeft(e->pos()) and selectionRect.setBottomRight(e->pos());

FED27

It works but the crop video in the second label is different from that one selected in the first label

frmmain.cpp
``
`
#include "frmmain.h"
#include "ui_frmmain.h"
#include<opencv2/highgui/highgui.hpp>

#include<QtCore>
#include<QMessageBox>

using namespace cv;
using namespace std;

// constructor ////////////////////////////////////////////////////////////////////////////////////
frmMain::frmMain(QWidget *parent) : QMainWindow(parent), ui(new Ui::frmMain) {
ui->setupUi(this);

capWebcam.open(0);              // associate the capture object to the default webcam

if(capWebcam.isOpened() == false) {                 // if unsuccessful
    QMessageBox::information(this, "", "error: capWebcam not accessed successfully \n\n exiting program\n");        // show error message
    exitProgram();                          // and exit program
    return;                                 //
}

qtimer = new QTimer(this);                                                      // instantiate timer
connect(qtimer, SIGNAL(timeout()), this, SLOT(processFrameAndUpdateGUI()));     // associate timer to processFrameAndUpdateGUI
qtimer->start(100);                  // start timer, set to cycle every 20 msec (50x per sec), it will not actually cycle this often

}

// destructor
frmMain::~frmMain() {
delete ui;
}

void frmMain::exitProgram() {
if(qtimer->isActive()) qtimer->stop(); // if timer is running, stop timer
QApplication::quit(); // and exit program
}

void frmMain::processFrameAndUpdateGUI() {
cv::Mat imgOriginal; // input image

bool blnFrameReadSuccessfully = capWebcam.read(imgOriginal);                    // get next frame from the webcam

if (!blnFrameReadSuccessfully || imgOriginal.empty()) {                            // if we did not get a frame
    QMessageBox::information(this, "", "unable to read from webcam \n\n exiting program\n");        // show error via message box
    exitProgram();                                                                              // and exit program
    return;                                                                                     //
}



flip(imgOriginal, imgOriginal, 0.5);
QImage qimgOriginal = convertOpenCVMatToQtQImage(imgOriginal);             // convert from OpenCV Mat to Qt QImage

ui->lblOriginal->setPixmap(QPixmap::fromImage(qimgOriginal)); // show images on form labels
ui->lblCanny->setPixmap(ui->lblOriginal->getRect(qimgOriginal,qimgOriginal.height(),qimgOriginal.width())); //
}

///////////////////////////////////////////////////////////////////////////////////////////////////
QImage frmMain::convertOpenCVMatToQtQImage(cv::Mat mat) {
if(mat.channels() == 1) { // if grayscale image
return QImage((uchar*)mat.data, mat.cols, mat.rows, mat.step, QImage::Format_Indexed8); // declare and return a QImage
} else if(mat.channels() == 3) { // if 3 channel color image
cv::cvtColor(mat, mat, CV_BGR2RGB); // invert BGR to RGB
return QImage((uchar*)mat.data, mat.cols, mat.rows, mat.step, QImage::Format_RGB888); // declare and return a QImage
} else {
qDebug() << "in convertOpenCVMatToQtQImage, image was not 1 channel or 3 channel, should never get here";
}
return QImage(); // return a blank QImage if the above did not work
}


my_qlabel.h

#ifndef WIDGET_H
#define WIDGET_H

#include <QtGui>
#include<QtCore>
#include <QtWidgets>
#include <QApplication>

class my_qlabel : public QLabel
{
public:
explicit my_qlabel(QWidget * parent = 0)
: QLabel(parent)
, mStartX(0)
, mStartY(0)
, mEndX(0)
, mEndY(0)
, mFirstClick(true)
, mPaintFlag(false)

{
    ;
}

void setPixmap(const QPixmap &);
QPixmap getRect(QImage original,int hv,int wv){
QPoint an,bn;

an.setX(a.x()*hv/width());
an.setY(a.y()*wv/height());

bn.setX(b.x()*hv/width());
bn.setY(b.y()*wv/height());



QRect selection;
selection.setRect(an.x(),an.y(),bn.x()-an.x(),bn.y()-an.y());

return  QPixmap::fromImage(original.copy(selection));

}

protected:
void mousePressEvent(QMouseEvent * e)
{
if(e->button() == Qt::LeftButton)
{
//store 1st point
if(mFirstClick)
{
// mFirstClick = false;
a=(e->pos());
qWarning() << "a "<< a.x()<< " "<< a.y();
qWarning() << "a "<< a.x()<< " "<< a.y();
b=(e->pos());
selectionRect.setRect(a.x(),a.y(),a.x()-b.x(),a.y()-b.y());
}

    }
}


void mouseMoveEvent(QMouseEvent *e){
   if  (mFirstClick)
   {
    b=(e->pos());
    qWarning() << "b "<< b.x()<< " "<< b.y();
    selectionRect.setRect(a.x(),a.y(),b.x()-a.x(),b.y()-a.y());
    repaint();
   }
}


void mouseRelaseEvent(QMouseEvent *e){
   mFirstClick=false;

}

void paintEvent(QPaintEvent * e)
{
    QLabel::paintEvent(e);

    if(mFirstClick)
    {
        QPainter painter(this);
        QPen paintpen(Qt::red);
        paintpen.setWidth(4);

        QPen linepen(Qt::black);
        painter.setPen(linepen);
        painter.drawRect(selectionRect);
    }
}

private:
int mStartX;
int mStartY;
int mEndX;
int mEndY;
QPoint a,b;
QRect selectionRect;
bool mFirstClick;
bool mPaintFlag;
};

#endif // WIDGET_H


my_qlabel.cpp

#include "my_qlabel.h"
#include <QPainter>
#include <QMouseEvent>
#include <QDebug>
#include <QAction>
#include <QFileDialog>
#include <iostream>
#include <QTextStream>

using namespace std;

void my_qlabel::setPixmap(const QPixmap& myPixmap){

QLabel::setPixmap(myPixmap.scaled(width(),height(),Qt::IgnoreAspectRatio,Qt::SmoothTransformation));

}

mrjj

Ok, but did you check the rect values?
are they what you expect ?
U are not very clear on "How it is different" and since you seems to
scale it also, im not sure what the actual issue is.

I would start checking the values for the QRect selectio and see if they make sense.

FED27

This is how i see the 2 label.
alt text

mrjj

HI
Super with pictures.

Is this how you set image on label 2?
void my_qlabel::setPixmap(const QPixmap& myPixmap){
QLabel::setPixmap(myPixmap.scaled(width(),height(),Qt::IgnoreAspectRatio,Qt::SmoothTransformation));
}

if yes, you ask it to scale to width(),height() and that would make it look zoomed as in your case.

FED27

No, i use this line

ui->lblCanny->setPixmap(ui->lblOriginal->getRect(qimgOriginal,qimgOriginal.height(),qimgOriginal.width()));

and this

QPixmap getRect(QImage original,int hv,int wv){
    QPoint an,bn;

    an.setX(a.x()*width()/hv);
    an.setY(a.y()*height()/wv);

    bn.setX(b.x()*width()/hv);
    bn.setY(b.y()*height()/wv);

//    an.setX(a.x());
//    an.setY(a.y());

//    bn.setX(b.x());
//    bn.setY(b.y());

    qWarning() << "an "<< an.x()<< " "<< an.y();
    qWarning() << "b "<< bn.x()<< " "<< bn.y();

    QRect selection;
    selection.setRect(an.x(),an.y(),bn.x()-an.x(),bn.y()-an.y());

    return  QPixmap::fromImage(original.copy(selection));


}

so the getRect() method return only a QPixmap

mrjj

ok. Not the scaling then.
Code seems fine.
Did you tried with fix area. like (0,0,100,100)
and then use debugger to follow through code ?

Should be easy to spot where it goes wrong rect wise.

FED27

i've solved. The image in the second label was not at the top left but centered, so part of the image was lost .Thank you

mrjj

@FED27
ah, sneaky :)
Good found.