Qt/QML : Send QImage From C++ to QML and Display The QImage On GUI

Question

I created a class Publisher which periodically emits a QImage object.

However I\'m having a tough time drawing the QImage to

Answer 1

QString getImage()
{
QByteArray byteArray;
QBuffer buffer(&byteArray);
buffer.open(QIODevice::WriteOnly);
img.save(&buffer,"JPEG");
 //save image data in string
QString image("data:image/jpg;base64,");
image.append(QString::fromLatin1(byteArray.toBase64().data()));
return image;
}

send image string directly to qml source

Answer 2

Use QQuickItem. There is a Qt example that does this.

C:\Qt\Examples\Qt-5.14.1\quick\scenegraph\threadedanimation

You make a class derived from QQuickItem and register it with qmlRegisterType. Provide the override function 'updatePaintNode' in your class. In the example, the class is 'Spinner'

In updatePaintNode:

Create a node class derived from QObject and QSGTransformNode

In the node class constructor:

Convert your Qimage to an QSGTexture with createTextureFromImage.

Create QSGSimpleTextureNode, set QSGTexture using setTexture

appendChildNode with QSGSimpleTextureNode

In QML add

import Spinner 1.0

and

Spinner {
    anchors.centerIn: parent
    anchors.horizontalCenterOffset: 80
    spinning: true
}

Answer 3

In other words, you have a class emitting a signal carrying a QImage and want to update an item in QML with that image? There are various solutions, none of which involves "converting a QImage to a QUrl" (whatever that means, surely you don't need to get a data URL carrying your image data...)

Use an image provider

This means you can use a plain Image item in your QML files.

1. Create a QQuickImageProvider subclass; give it a QImage member (the image to provider), override requestImage to provide that image (the actual id requested does not really matter, see below), and a slot that receives a QImage and updates the member.
2. Connect your Publisher signal to your provider's slot
3. Install the provider into the QML engine via QQmlEngine::addImageProvider (see QQuickView::engine); again the id does not really matter, just use a sensible one

4. In QML, just use a plain Image element with a source like this

   Image {
       id: myImage
       source: "image://providerIdPassedToAddImageProvider/foobar"
   }
   

   foobar will be passed to your provider, but again, it doesn't really matter.

5. We're almost there, we now only need a way to push the image updates to the QML world (otherwise Image will never know when to update itself). See my answer here for how to do that with a Connections element and a bit of JS.

   Note that in general you don't need to make Publisher a QML type, you just need to create one instance in C++ and expose it to the QML world via QQmlContext::setContextProperty.

Use a custom Qt Quick 2 Item

QQuickPaintedItem is probably the most convenient for the job as it offers a paint method taking a QPainter. Hence the big plan is

1. Subclass QQuickPaintedItem: the subclass stores the QImage to be painted and has a slot that sets the new QImage. Also its paint implementation simply paints the image using QPainter::drawImage.
2. Expose the subclass to the QML world via qmlRegisterType (so that you can use it in QML)

3. Figure out a way to connect the signal carrying the new image to the items' slot.

   This might be the tricky part.

   To perform the connection in C++ you need a way to figure out that the item has been created (and get a pointer to it); usually one does this by means of assigning the objectName property to some value, then using findChild on the root object (as returned by QQuickView::rootObject()) to get a pointer to the item itself. Then you can use connect as usual.

   Or, could instead perform the connection in QML, just like above, via a Connections element on the publisher C++ object exposed to the QML world:

   MyItem {
       id: myItem
   }        
   
   Connections {
       target: thePublisherObjectExposedFromC++
       onNewImage: myItem.setImage(image)
   }
   

   This has the advantage of working no matter when you create the MyItem instance; but I'm not 100% sure it will work because I'm not sure you can handle the QImage type in QML.

Answer 4

When I've had image-producing C++ classes I've wanted to embed in QML, I've always done it by making the C++ class a subclass of QDeclarativeItem (there'll be a new QtQuick 2.0 equivalent of course), overriding the paint method with the appropriate drawing code, which maybe as simple as

void MyItem::paint(QPainter* painter,const QStyleOptionGraphicsItem*,QWidget*) {
  painter->drawImage(QPointF(0.0f,0.0f),_image);
}

if you have a QImage of the right size already... and Job Done. For animation, just ping update() when there's something new to draw.