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

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.