Understanding QGraphicsScene in Qt - qt

Question

I am trying to understand the basics of Qt. After going through some posts, I came to know that ui_mainwindow.h gets created by UIC tool and that ui_mainwindow.h contains the information about my form/ui that I created.
In my GUI, I have taken a pushbutton and a graphics view. I want a simple image (which I am creating inside the program itself) gets displayed in the graphicsView. I am trying to do it with two ways (for learning purpose):
I can write the code inside on_pushButton_clicked()(i.e. the slot of my push_button).
I am trying to put the image from the main()
Problem: I am done with the first method. I used the following lines of code inside on_pushButton_clicked() and it worked.
void MainWindow::on_pushButton_clicked()
{
//Display image in the graphics viewer
Mat img(200,200, CV_8UC3, Scalar(255,0,0));
QImage image( img.data, img.cols, img.rows, img.step, QImage::Format_RGB888 );
QGraphicsScene* scene = new QGraphicsScene();
QGraphicsPixmapItem* item = new QGraphicsPixmapItem(QPixmap::fromImage(image));
scene->addItem(item);
ui->graphicsView->setScene(scene);
}
Now, I want to do the similar thing from the main(). To do that, now my main() looks like following:
#include "mainwindow.h"
#include <QApplication>
//For image
#include <QImage>
#include <QPixmap>
#include <QGraphicsPixmapItem>
//#include "ui_mainwindow.h"
//OPENCV Headers
#include <opencv2/opencv.hpp>
#include <opencv2/core.hpp>
#include <opencv2/imgcodecs/imgcodecs.hpp>
using namespace cv;
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainWindow w;
//Display image in the graphics viewer
Mat img(200,200, CV_8UC3, Scalar(255,0,0));
QImage image( img.data, img.cols, img.rows, img.step, QImage::Format_RGB888 );
QGraphicsScene* scene = new QGraphicsScene();
QGraphicsPixmapItem* item = new QGraphicsPixmapItem(QPixmap::fromImage(image));
scene->addItem(item);
w.ui->graphicsView->setScene(scene);
w.show();
return a.exec();
}
The above code written inside the main() works if I put #include "ui_mainwindow.h" in main.cpp. But if I comment #include "ui_mainwindow.h" and w.ui->graphicsView->setScene(scene); then, it throws error for the QGraphicsScene* scene = new QGraphicsScene();.
Error is main.cpp:32: error: allocation of incomplete type 'QGraphicsScene' QGraphicsScene* scene = new QGraphicsScene();
QUESTIONS: Why is the connection between QGraphicsScene and "ui_mainwindow.h". I understand that I need "ui_mainwindow.h" for the line w.ui->graphicsView->setScene(scene); becasue there I am using my ui but I don't understand the need for QGraphicsScene.

Answer 1

If You want to draw only static image better use just QLabel and set in some image
QImage *image = new QImage (":/prefix/image/vl.jpg" );
QLabel *magelab = new QLabel();
magelab->setPixmap(QPixmap::fromImage(*image));
Generaly QGraphicsScene better using with connection QGraphicsView. That is mean QGraphicsView set some object (scene) from QGraphicsScene just like:
class SomeObject : public QGraphicsView
{
Q_OBJECT
public:
explicit Schemat(QWidget *parent = 0);
private:
QGraphicsScene *scene;
};
and source
Schemat::Schemat( QWidget *parent) : QGraphicsView(parent)
{
scene = new QGraphicsScene(this);
this->setScene(scene);
// to Your scene You can add some image for example
scene->addPixmap(SOME_PIXMAP)
}
Then You create main window and add with Your QGraphicsView, for example as some part QGroupBox
void MainWindow::SetupSchemat()
{
schema = new Schemat();
QGroupBox *schbox;
QHBoxLayout *hschbox;
schbox = new QGroupBox(this);
hschbox = new QHBoxLayout(this);
schbox->setTitle("SomeScene");
hschbox->addWidget(schema); //add scene to layout
schbox->setLayout(hschbox);
}
QGraphicsScene is like some part of Your MainWindow on which you can make some animation, You can something draw. QGraphicsScene is more better to using if You want use animation not only static image it supplies more option to manipulate image (ex scaling, catch mouse click, manage via cordinates others object), and other object each should be animate or just display. To QGraphicsScene You can add some QGraphicsItem in turn each QGraphicsItem can moving onto QGraphicsScene with particular conditions defined erly or in flow. Together QGraphicsView, QGraphicsScene and QGraphicsItem can created animation or just image in some part Your main window.
Also nice explained You will find here
https://www.youtube.com/watch?v=fmSs2mNGh9I