How to display a QPixmap/QImage in QML?

[A Former User]

@KroMignon Hi! Here is a skeleton for a simple approach:

pixmapcontainer.h - pass around objects of this type between C++ and QML

#ifndef PIXMAPCONTAINER_H
#define PIXMAPCONTAINER_H

#include <QObject>
#include <QPixmap>

class PixmapContainer : public QObject
{
    Q_OBJECT
public:
    explicit PixmapContainer(QObject *parent = 0);
    QPixmap pixmap;
};

#endif // PIXMAPCONTAINER_H

backend.h - provides the pixmaps

#ifndef BACKEND_H
#define BACKEND_H

#include <QObject>

class Backend : public QObject
{
    Q_OBJECT
public:
    explicit Backend(QObject *parent = 0);
    Q_INVOKABLE QObject* getPixmapContainer() const;
};

#endif // BACKEND_H

backend.cpp

#include "backend.h"
#include <QQmlEngine>
#include "pixmapcontainer.h"

Backend::Backend(QObject *parent) :
    QObject(parent)
{
}

QObject *Backend::getPixmapContainer() const
{
    PixmapContainer * pc = new PixmapContainer();
    pc->pixmap.load("/path/to/image.jpg");
    Q_ASSERT(!pc->pixmap.isNull());
    QQmlEngine::setObjectOwnership(pc, QQmlEngine::JavaScriptOwnership);
    return pc;
}

pixmapimage.h - QtQuick component to show pixmaps

#ifndef PIXMAPIMAGE_H
#define PIXMAPIMAGE_H

#include <QQuickPaintedItem>
#include <QPainter>
#include "pixmapcontainer.h"

class PixmapImage : public QQuickPaintedItem
{
    Q_OBJECT
public:
    PixmapImage(QQuickItem *parent = Q_NULLPTR);
    Q_INVOKABLE void setImage(QObject *pixmapContainer);

protected:
    virtual void paint(QPainter *painter) Q_DECL_OVERRIDE;

private:
    PixmapContainer m_pixmapContainer;
};

#endif // PIXMAPIMAGE_H

pixmapimage.cpp

#include "pixmapimage.h"

PixmapImage::PixmapImage(QQuickItem *parent) :
    QQuickPaintedItem(parent)
{
}

void PixmapImage::setImage(QObject *pixmapContainer)
{
    PixmapContainer *pc = qobject_cast<PixmapContainer*>(pixmapContainer);
    Q_ASSERT(pc);
    m_pixmapContainer.pixmap = pc->pixmap;
    update();
}

void PixmapImage::paint(QPainter *painter)
{
    painter->drawPixmap(0, 0, width(), height(), m_pixmapContainer.pixmap);
}

main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>

#include "pixmapcontainer.h"
#include "backend.h"
#include "pixmapimage.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    qmlRegisterType<PixmapContainer>("io.qt.forum", 1, 0, "PixmapContainer");
    qmlRegisterType<Backend>("io.qt.forum", 1, 0, "Backend");
    qmlRegisterType<PixmapImage>("io.qt.forum", 1, 0, "PixmapImage");

    QQmlApplicationEngine engine;
    Backend backend;
    engine.rootContext()->setContextProperty("backend", &backend);
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    return app.exec();
}

main.qml

import QtQuick 2.7
import QtQuick.Controls 1.5
import io.qt.forum 1.0

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    PixmapImage {
        id: pixmapImage
        anchors.fill: parent
    }

    Button {
        text: "Get Pixmap"
        onClicked: pixmapImage.setImage( backend.getPixmapContainer() )
    }
}

[KroMignon]

@Wieland Thanks a lot for your code example. This was very interresting, but still to much overhead for my very simple use case!

I found an alternative solution, which works fine for me. I post it as reply to my question if someone else has the same needs.
To pass the QImage to QML, I simply serialize into a QByteArray like this:

Q_PROPERTY(QString picture READ picture CONSTANT)

...

const QString MyImageClasse::picture()
{
   QImage myImage;
  // Some init code to setup the image (e.g. loading a PGN/JPEG, etc.)
  QByteArray bArray;
  QBuffer buffer(&bArray);
  buffer.open(QIODevice::WriteOnly);
  myImage.save(&buffer, "JPEG");

  QString image("data:image/jpg;base64,");
  image.append(QString::fromLatin1(bArray.toBase64().data()));

  return image;
}

...

Works for me (TM) ;-)

Regards

[Julian Guarin]

@Wieland said in How to display a QPixmap/QImage in QML?:

qmlRegisterType<PixmapContainer>("io.qt.forum", 1, 0, "PixmapContainer");
qmlRegisterType<Backend>("io.qt.forum", 1, 0, "Backend");
qmlRegisterType<PixmapImage>("io.qt.forum", 1, 0, "PixmapImage");

@Wieland Why did you register the backend & container objects? Why would you access such objects in the QML? Just being curious...

[A Former User]

@Julian-Guarin You're right, registering those wasn't necessary here.

[Mickey Rockman]

This is a very good example. I follow the code and it works exactly as I expected.
Thank you so much

[Yunus]

Hi, I just copied this example but I got some errors. in "backend.cpp" and "pixmapimage.cpp" . The errors are the same and:

error: undefined reference to `PixmapContainer::PixmapContainer(QObject*)'

What is the problem?

[jsulm]

@yunus said in How to display a QPixmap/QImage in QML?:

What is the problem?

The problem is right there in the error message.
Where is PixmapContainer::PixmapContainer(QObject*) defined?
I guess you forgot to add the cpp file for PixmapContainer.

Or change this line

explicit PixmapContainer(QObject *parent = 0);

to

explicit PixmapContainer(QObject *parent = 0) : QObject(parent) {}

[Yunus]

@jsulm yes I didnt add it because in the example there is no file named as pixmapcontainer.cpp

[Yunus]

@jsulm Ahh okey. After added the line you advised it worked thak you so much

[Huang Zhen]

@KroMignon It works, thank you!

[fcarney]

@KroMignon said in How to display a QPixmap/QImage in QML?:

I found an alternative solution

So are you feeding that string to the "source" property of QML Image?

[KroMignon]

@fcarney said in How to display a QPixmap/QImage in QML?:

So are you feeding that string to the "source" property of QML Image?

Yes

[JoeMaesai]

Thank a lot for your code example. Really appreciate.