How to draw on Image loaded in a QGraphicsView



  • @Nico1564
    It depends on what you want to draw on top. If you drew the image in a QGraphicsBitmapItem and you want to draw primitives like lines or text etc. on top, just draw them in the correct position after you draw the bitmap. The draw order will take car of it.
    Or, make the bitmap item the parent of the items you want to draw on top i.e. children are draw after parents unless you change that behaviour.
    You should really show people the code you use to draw the bitmap, don't need your whole code base :-)



  • Hi SGaist and kenchan, thank you for answer

    I want (first) draw some basic things like a circle and a line. Next i want to "draw" an other image more smaller than the first one and it have to move on the first picture.

    Here is the code of the class :

    #rOg_image.cpp
    #include "rOg_image.h"

    // Constructor of the class, create the scene and set default parameters
    rOg_image::rOg_image(bool isContextualMenu, QWidget * parent) :
    QGraphicsView(parent)
    {
    // Set default zoom factors
    zoomFactor=DEFAULT_ZOOM_FACTOR;
    zoomCtrlFactor=DEFAULT_ZOOM_CTRL_FACTOR;

    // Create the scene
    scene = new QGraphicsScene();
    
    // Allow mouse tracking even if no button is pressed
    this->setMouseTracking(true);
    
    // Add the scene to the QGraphicsView
    this->setScene(scene);
    
    // Update all the view port when needed, otherwise, the drawInViewPort may experience trouble
    this->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
    
    // When zooming, the view stay centered over the mouse
    this->setTransformationAnchor(QGraphicsView::AnchorUnderMouse);
    
    this->setResizeAnchor(QGraphicsView::AnchorViewCenter);
    
    // Initialize contextual menu if requested
    if (isContextualMenu)
    {
        setContextMenuPolicy(Qt::CustomContextMenu);
        connect(this, SIGNAL(customContextMenuRequested(const QPoint&)), this, SLOT(showContextMenu(const QPoint&)));
    }
    
    // Disable scroll bar to avoid a unwanted resize recursion
    

    // setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    // setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);

    // Add the default pixmap at startup
    pixmapItem = scene->addPixmap(pixmap);
    

    }

    // Destructor of the class
    rOg_image::~rOg_image()
    {
    delete pixmapItem;
    delete scene;
    }

    // Display contextual menu
    void rOg_image::showContextMenu(const QPoint & pos)
    {
    // Get the mouse position in the scene
    QPoint globalPos = mapToGlobal(pos);
    // Create the menu and add action
    QMenu contextMenu;
    contextMenu.addAction("Reset view", this, SLOT(fitImage()));
    // Display the menu
    contextMenu.exec(globalPos);
    }

    // Set or update the image in the scene
    void rOg_image::setImage(const QImage & image)
    {
    // Update the pixmap in the scene
    pixmap=QPixmap::fromImage(image);
    pixmapItem->setPixmap(pixmap);

    // Resize the scene (needed is the new image is smaller)
    scene->setSceneRect(QRect (QPoint(0,0),image.size()));
    
    // Store the image size
    imageSize = image.size();
    

    }

    // Set an image from raw data
    void rOg_image::setImageFromRawData(const uchar * data, int width, int height, bool mirrorHorizontally, bool mirrorVertically)
    {
    // Convert data into QImage
    QImage image(data, width, height, width*3, QImage::Format_RGB888);

    // Update the pixmap in the scene
    pixmap=QPixmap::fromImage(image.mirrored(mirrorHorizontally,mirrorVertically));
    pixmapItem->setPixmap(pixmap);
    
    // Resize the scene (needed is the new image is smaller)
    scene->setSceneRect(QRect (QPoint(0,0),image.size()));
    
    // Store the image size
    imageSize = image.size();
    

    }

    // Fit the image in the widget
    void rOg_image::fitImage()
    {
    // Get current scroll bar policy
    Qt::ScrollBarPolicy currentHorizontalPolicy = horizontalScrollBarPolicy();
    Qt::ScrollBarPolicy currentverticalPolicy = verticalScrollBarPolicy();

    // Disable scroll bar to avoid a margin around the image
    setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
    
    // Fit the scene in the QGraphicsView
    this->fitInView(scene->sceneRect(), Qt::KeepAspectRatio);
    
    // Restaure scroll bar policy
    setHorizontalScrollBarPolicy(currentHorizontalPolicy);
    setVerticalScrollBarPolicy(currentverticalPolicy);
    

    }

    // Called when a mouse button is pressed
    void rOg_image::mousePressEvent(QMouseEvent *event)
    {
    // Drag mode : change the cursor's shape
    if (event->button() == Qt::LeftButton) this->setDragMode(QGraphicsView::ScrollHandDrag);
    // if (event->button() == Qt::RightButton) this->fitImage();
    QGraphicsView::mousePressEvent(event);
    }

    // Called when a mouse button is pressed
    void rOg_image::mouseReleaseEvent(QMouseEvent *event)
    {
    // Exit drag mode : change the cursor's shape
    if (event->button() == Qt::LeftButton) this->setDragMode(QGraphicsView::NoDrag);
    QGraphicsView::mouseReleaseEvent(event);
    }

    #ifndef QT_NO_WHEELEVENT

    // Call when there is a scroll event (zoom in or zoom out)
    void rOg_image::wheelEvent(QWheelEvent *event)
    {
    // When zooming, the view stay centered over the mouse
    this->setTransformationAnchor(QGraphicsView::AnchorUnderMouse);

    double factor = (event->modifiers() & Qt::ControlModifier) ? zoomCtrlFactor : zoomFactor;
    if(event->delta() > 0)
        // Zoom in
        scale(factor, factor);
    else
        // Zooming out
        scale(1.0 / factor, 1.0 / factor);
    
    // The event is processed
    event->accept();
    

    }

    #endif

    // Overload the mouse MoveEvent to display the tool tip
    void rOg_image::mouseMoveEvent(QMouseEvent *event)
    {
    // Get the coordinates of the mouse in the scene
    QPointF imagePoint = mapToScene(QPoint(event->x(), event->y() ));
    // Call the function that create the tool tip
    setToolTip(setToolTipText(QPoint((int)imagePoint.x(),(int)imagePoint.y())));
    // Call the parent's function (for dragging)
    QGraphicsView::mouseMoveEvent(event);
    }

    // Overload the function to draw over the image
    void rOg_image::drawForeground(QPainter *painter, const QRectF&)
    {

    // Call the function to draw over the image
    this->drawOnImage(painter,imageSize);
    
    // Reset transformation and call the function draw in the view port
    painter->resetTransform();
    
    // Call the function to draw in the view port
    this->drawInViewPort(painter, this->viewport()->size());
    

    }

    // Overloaded functionthat catch the resize event
    void rOg_image::resizeEvent(QResizeEvent *event)
    {
    // First call, the scene is created
    if(event->oldSize().width() == -1 || event->oldSize().height() == -1) return;

    // Get the previous rectangle of the scene in the viewport
    QPointF P1=mapToScene(QPoint(0,0));
    QPointF P2=mapToScene(QPoint(event->oldSize().width(),event->oldSize().height()));
    
    // Stretch the rectangle around the scene
    if (P1.x()<0) P1.setX(0);
    if (P1.y()<0) P1.setY(0);
    if (P2.x()>scene->width()) P2.setX(scene->width());
    if (P2.y()>scene->height()) P2.setY(scene->height());
    
    // Fit the previous area in the scene
    this->fitInView(QRect(P1.toPoint(),P2.toPoint()),Qt::KeepAspectRatio);
    

    }

    // Define the virtual function to avoid the "unused parameter" warning
    QString rOg_image::setToolTipText(QPoint imageCoordinates)
    {
    (void)imageCoordinates;
    return QString("");
    }

    // Define the virtual function to avoid the "unused parameter" warning
    void rOg_image::drawOnImage(QPainter* , QSize )
    {}

    // Define the virtual function to avoid the "unused parameter" warning
    void rOg_image::drawInViewPort(QPainter* , QSize )
    {}

    in my mainwindow (called QtGuiApplication1.cpp) i create an instance with the next lines :

    imgWidget = new rOg_image(this);

    #ifdef LOAD_RAW_RGB_DATA

    // Prepare raw image data, format is RGBRGBRGB...
    data = new uchar[WIDTH*HEIGHT * 3];
    int index = 0;
    for (int y = 0;y<HEIGHT;y++)
    	for (int x = 0;x<WIDTH;x++)
    	{
    		data[index++] = 0xFF * (x<(2.*WIDTH / 3.)); // || 0x00;
    		data[index++] = 0xFF * (x>(WIDTH / 3.) && (x<(2 * WIDTH / 3.)));
    		data[index++] = 0xFF * (x>(WIDTH / 3.));
    	}
    // Draw the raw image in the image widget
    imgWidget->setImageFromRawData(data, WIDTH, HEIGHT);
    

    #endif

    // Set background color (gray)
    imgWidget->setBackgroundBrush(QBrush(QColor(0x7F, 0x7F, 0x7F)));


  • @Nico1564
    OK, so what I understand from your code is, you are adding an image to the scene in the constructor of your QGraphicsView. Then, you add another image to the scene later, correct?
    BTW, when you add the second image you do not remove the old one but you overwrite the pointer to the first pixmap item in your class, is this deliberate?

    So, the problem is what, you can't see the second image only the first one?
    I don't see why you have to over load the drawForeground() function but I guess that came with the code you downloaded?
    So your first image is a kind of background for your later drawing?



  • @kenchan this code is only the code that I have download. And yes in fact, there is 2 images, 1 is a grey background (set by the foncion setImageFromRawData) and the second is and image that i choose with this code in the QtGuiApplication1.cpp :

            QString fichier = QFileDialog::getOpenFileName(this, "Ouvrir un fichier", QString(), "Images (*.png *.gif *.jpg *.jpeg)");
    	if (fichier.isEmpty())
    	{
    		QMessageBox::critical(this, "Erreur", "Erreur dans la selection de l'image");
    	}
    	else
    	{
    		QMessageBox::information(this, "Fichier", "Vous avez selectionne :\n" + fichier);
    		imgWidget->setImage(QImage(fichier));
    		ui.horizontalLayout->addWidget(imgWidget);
    

    ui.horizontalLayout is a Layout in my MainWindow

    Do you think that I can put my "draws" converted in QImages with the same type of lines ?



  • @Nico1564
    I don't understand what you mean by this:
    "Do you think that I can put my "draws" converted in QImages with the same type of lines ?"
    You should be able to just draw on top of previously dawn items. That is the natural draw order for a QGraphicsScene. I can't see why it will not do this for you. I must look more closely at your code.
    Can you post a screen shot of how it appears now?



  • in fact i just begin the c++ programming, and i'm a little lost. I know how to draw but in this case, i don't know where because i don't want to draw in the scene but on the (real) Image (not the background)

    What i wanted to say is that, if i recall SetImage(....) in my QtGuiApplication1, does the previous picture will be replaced by the new one or not ?

    I could not upload all my code for confidentials reason, so let me some times to hide somes lines pls



  • @Nico1564
    Ah, you say you don't want to draw in the scene but draw something on the actual image and then put that in the scene?
    OK, you could make a painter draw on the image then draw other things on the image using the painter. Then you could draw the resulting image in the scene, if that is what you mean.
    have a function to draw on the image something like this, then put the image in the scene.

        QImage image(512,512,QImage::Format_RGBA8888);
        image.fill(QColor(128,128,128));
        QPainter painter(&image);
        //...
        //draw something on the image
        painter.drawRect(200,200,200,200);
        //...
        painter.end();
    


  • Ok so to do that, i should modify this part of the QtGuiApplication.cpp ?

    	QString fichier = QFileDialog::getOpenFileName(this, "Ouvrir un fichier", QString(), "Images (*.png *.gif *.jpg *.jpeg)");
    	if (fichier.isEmpty())
    	{
    		QMessageBox::critical(this, "Erreur", "Erreur dans la selection de l'image");
    	}
    	else
    	{
    		QMessageBox::information(this, "Fichier", "Vous avez selectionne :\n" + fichier);
    		//create painter with fichier then draw my circle on this painter
    		imgWidget->setImage(QImage(fichier));
    		ui.horizontalLayout->addWidget(imgWidget);
    		defCoos();
    	}
    

    Another question on my circle, once I have drawn it, I could not delete it right ? Or there is a system of layer with QPainter or QImage like we should find in Photoshop for exemple?



  • @Nico1564
    I just edited my last reply with some code to paint on an image. Use an image format which works best for your needs.
    No, using a painter you can't delete only overwrite. That is the difference between a painter and a scene.
    If you manage your own primitives you can repaint the image when you need to I suppose.
    They are low level things you are supposed to add the high level stuff like a Photoshop app does :-)



  • @Nico1564
    I just built that sample you used. If you draw something with the painter in the drawOnImage() function you will see it drawn on top of the image.



  • @kenchan yes i just test it but it overwrite the first image (wich is a map, let's call it like this if you agree), so i have to set the backgroud of the draw to transparent
    EDIT : even setting the background transparent, the map disappear



  • @Nico1564
    Hm, well i just made the sample load the Qt image in the constructor. Then drew a rectangle with the painter in the drawOnImage() functions. That function gets called from the drawForeground() function so it gets refreshed all the time.
    You draw another image but that should get over written by what ever is drawn in the drawOnImage(). Now, I am not sure when you are calling the setBackgroundBrush() function but I would suggest you only call that before you do any other drawing and not after, just to be on the safe side.



  • @Nico1564 said in How to draw on Image loaded in a QGraphicsView:

    Ok so to do that, i should modify this part of the QtGuiApplication.cpp ?

    	QString fichier = QFileDialog::getOpenFileName(this, "Ouvrir un fichier", QString(), "Images (*.png *.gif *.jpg *.jpeg)");
    	if (fichier.isEmpty())
    	{
    		QMessageBox::critical(this, "Erreur", "Erreur dans la selection de l'image");
    	}
    	else
    	{
    		QMessageBox::information(this, "Fichier", "Vous avez selectionne :\n" + fichier);
    		//create painter with fichier then draw my circle on this painter
    		imgWidget->setImage(QImage(fichier));
    		ui.horizontalLayout->addWidget(imgWidget);
    		defCoos();
    	}
    

    Another question on my circle, once I have drawn it, I could not delete it right ? Or there is a system of layer with QPainter or QImage like we should find in Photoshop for exemple?

    If you want to work with something like layers, then you should not modify the image. GraphicsView is a framework that allows you to create multiple visual objects (images, circles, rectangles, texts,...) and manipulate them individually. You can move objects along x/y axis, you can define a z-order (which object is drawn on top, which below, etc.), and you can define transformations (scaling, rotating,...). You can group objects to they can be manipulated together. A bit like Powerpoint, really, but with code.

    For what you want, it would make sense to create the image, and then a separate circle (QGraphicsEllipseItem) on top of it. Want to delete the circle later on? Just delete the QGraphicsEllipseItem, and your image stays.

    I recommend reading about the basics of the QGraphicsView framework: https://doc.qt.io/qt-5/graphicsview.html



  • Ok I have understand (i think) so I just try something in a new project (to see if i have really understood) but I failed and now I'm depressed .... :(

    here is my new code :

    testGraphicView.cpp

    #include "testGraphicView.h"
    
    testGraphicView::testGraphicView(QWidget *parent)
    	: QMainWindow(parent)
    {
    	ui.setupUi(this);
    	scene = new QGraphicsScene(this);
    	ui.graphicsView->setScene(scene);
    	connect(ui.pushButton, SIGNAL(triggered), this, SLOT(Circle()));
    }
    
    void testGraphicView::Circle()
    {
    	ellipse = scene->addEllipse(10, 10, 100, 100);
    
    }
    

    testGraphicView.h

    #pragma once
    
    #include <QtWidgets/QMainWindow>
    #include "ui_testGraphicView.h"
    #include <QGraphicsView>
    
    
    class testGraphicView : public QMainWindow
    {
    	Q_OBJECT
    
    public:
    	testGraphicView(QWidget *parent = Q_NULLPTR);
    
    public slots :
    	void Circle();
    
    private:
    	Ui::testGraphicViewClass ui;
    	QGraphicsScene *scene;
    	QGraphicsEllipseItem *ellipse;
    
    };
    


  • @Nico1564 No no, That is not going to work. A QMainWindow is not going to work because it is not a QGraphicsView.
    You must but your QGraphicsView in the central widget of the main window, like the sample you used did.
    The sample is not a bad start but you should simplify it by just removing the functions you don't need for now.



  • @Nico1564
    I would suggest you don't call the drawOnImage(painter,imageSize); function.
    Don't add the initial image, just the one you want to add the way you want to add it.
    Add a function to your rOg_image to add the circle or whatever you want to draw on your image, or just draw it directly with the scene functions. Draw it after you have loaded your image.
    Call the draw function you made after you load and draw the image.
    Then you should see what ever you draw on top of the image and you will be able to zoom and scroll after you have zoomed in on the scene.
    Please do as @Asperamanca suggested and read up on the docs for the QGraphicsView and QGraphicsScene.



  • Here's a minimal GraphicsView sample, that draws an Ellipse:

    mainwindow.h

    #ifndef MAINWINDOW_H
    #define MAINWINDOW_H
    
    #include <QWidget>
    #include <QGraphicsView>
    #include <QGraphicsScene>
    
    class MainWindow : public QWidget
    {
        Q_OBJECT
    
    public:
        MainWindow(QWidget *parent = 0);
        ~MainWindow();
    
    private:
        QGraphicsScene* m_pScene;
        QGraphicsView* m_pView;
    };
    
    #endif // MAINWINDOW_H
    

    mainwindow.cpp

    #include "mainwindow.h"
    #include <QGraphicsEllipseItem>
    
    MainWindow::MainWindow(QWidget *parent)
        : QWidget(parent)
    {
        m_pScene = new QGraphicsScene(0,0,800,480,this);
    
        m_pView = new QGraphicsView(m_pScene, this);
        m_pView->setFrameStyle(QFrame::NoFrame);
        m_pView->setGeometry(0,0,800,480);
        m_pView->setAutoFillBackground(false);
        m_pView->show();
    
        // Add your image here, as a QGraphicsPixmapItem, and it will be drawn below the ellipse
        QGraphicsEllipseItem* testItem = new QGraphicsEllipseItem(20,30,120,70,0);
        m_pScene->addItem(testItem);
    }
    
    MainWindow::~MainWindow()
    {
    }
    

    main.cpp

    #include <QApplication>
    #include "mainwindow.h"
    
    int main(int argc, char *argv[])
    {
        QApplication a(argc, argv);
        MainWindow w;
        w.show();
    
        return a.exec();
    }
    

    Maybe you can go from there



  • Yeah i know that this work but i need to draw with fonctions. I successed to do that but I don't want to have the GraphicsView at the beginning of the program, and i want to add draws like an Ellipse (and the image) when i click on buttons.



  • @Nico1564
    Because you have a main window you can add menus, toolbars with buttons or dock widget with buttons. You can make these buttons and menus call your daw functions or what ever you want.
    The sample you used is a good basis to start from. the sample that @Asperamanca has posted is also a good basis to start from. Now it is up to you to study the docs for the gui elements and do what you need to do.



  • @Nico1564 said in How to draw on Image loaded in a QGraphicsView:

    Yeah i know that this work but i need to draw with fonctions. I successed to do that but I don't want to have the GraphicsView at the beginning of the program, and i want to add draws like an Ellipse (and the image) when i click on buttons.

    These lines could just as well happen inside a button click

        QGraphicsEllipseItem* testItem = new QGraphicsEllipseItem(20,30,120,70,0);
        m_pScene->addItem(testItem);
    

    In that case, you would create an ellipse every time you pressed that button.

    I don't quite understand the "I don't want to have the GraphicsView at the beginning of the program" - maybe you could elaborate on that.



Looks like your connection to Qt Forum was lost, please wait while we try to reconnect.