Qt/QML : Send QImage From C++ to QML and Display The QImage On GUI
Solution 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.
- Create a
QQuickImageProvider
subclass; give it aQImage
member (the image to provider), overriderequestImage
to provide that image (the actualid
requested does not really matter, see below), and a slot that receives aQImage
and updates the member. - Connect your
Publisher
signal to your provider's slot - Install the provider into the QML engine via
QQmlEngine::addImageProvider
(seeQQuickView::engine
); again theid
does not really matter, just use a sensible one -
In QML, just use a plain
Image
element with a source like thisImage { id: myImage source: "image://providerIdPassedToAddImageProvider/foobar" }
foobar
will be passed to your provider, but again, it doesn't really matter. -
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 viaQQmlContext::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
- Subclass
QQuickPaintedItem
: the subclass stores theQImage
to be painted and has a slot that sets the new QImage. Also itspaint
implementation simply paints the image usingQPainter::drawImage
. - Expose the subclass to the QML world via
qmlRegisterType
(so that you can use it in QML) -
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 usingfindChild
on the root object (as returned byQQuickView::rootObject()
) to get a pointer to the item itself. Then you can useconnect
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.
Solution 2
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.
Solution 3
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
trianta2
BSEE & MSCS. Working on ML/AI applications at AT&T Labs Research.
Updated on December 13, 2020Comments
-
trianta2 over 3 years
I created a class
Publisher
which periodically emits aQImage
object.However I'm having a tough time drawing the
QImage
to a QML element. It appears that theImage
andCanvas
QML components require aQUrl
instead of aQImage
, but I'm not sure how to convert myQImage
to aQUrl
. Edit4: When I say QUrl, I don't mean I'm trying to convert an image to a URL. That's nonsense. I mean I want to generate a reference to this image, which is not on disk, and the data type that QML components are asking for is a URL.I've done some research and found that
QQuickImageProvider
provides a solution, but I haven't found any documentation explaining how to convert myQImage
signal to aQUrl
that I can use for drawing. Any example code or reference documentation would be appreciated.Thanks for your help!
Edit1:
I've taken a look here: http://qt-project.org/doc/qt-5.0/qtquick/qquickimageprovider.html and I do not see how I pass a QImage to the quick image provider and from it create a QUrl.
Edit2. Here is the header. The implementation should not be important.
class Publisher { Q_OBJECT public: Publisher(QObject* parent = 0); virtual ~Publisher(void); Q_SIGNALS: void newImage(const QImage& newImage); };
Edit 3. Here is my QML code, but I don't know how to draw my QImage, so this code is kind of meaningless.
my main.cpp file:
int main(int argc, char *argv[]) { QGuiApplication app(argc, argv); qmlRegisterType<Publisher>("Components", 1, 0, "Publisher"); QtQuick2ApplicationViewer viewer; viewer.setMainQmlFile(QStringLiteral("qml/QQuickViewExample/main.qml")); viewer.showExpanded(); return app.exec(); }
my main.qml file:
import QtQuick 2.0 import Components 1.0 Rectangle { id : testRect width: 360 height: 360 Image{ anchors.fill: parent id: myImage Publisher { id: myPub onNewImage: { myImage.source = newImage; #I know this doesnt work, it needs a QUrl and not a QImage } } } }
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peppe over 10 yearsNote that OP seems to be asking about Qt Quick 2, your solution is QtQuick 1 based. The closest fit is
QQuickPaintedItem
. -
timday over 9 yearsFor the record, I've just used the second technique suggested above in Qt5.4 successfully: to wire one C++ QtQuick item's QImage property to another C++ class which is QQuickPainter subclass and exists for the sole purpose of displaying QImage's. I don't even need the connections, in the QML MyItem {image: otherItem.image;} binding works fine when otherItem emits an imageChanged.
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Daniel Saner over 9 yearsMethod 1 doesn't seem to work, because
QQuickImageProvider
is not aQObject
, and therefore cannot have any slots. Using multiple inheritance to give it slots anyway seems to be a bad idea as well; MOC doesn't like it. -
S.M.Mousavi about 8 years
QQuickPaintedItem
is related to old painting method. For new technology and most performance, you need to useQQuickItem
and use texture. -
xtofl over 3 yearsThis will probably work, but is knowlngly wasting resources. Therefor I must downvote; the planet has too much CO2 already.
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Stephen Quan almost 3 yearsI, independently came up with the same solution, but, I used PNG instead of JPEG because it was lossless.