QML How to paint different to grabToImage()

Question

Is there any possible way to make grabToImage() buffer different from what's displayed in GUI? I'm using ChartView without legend in GUI and want to export it to PNG with legend. So I try:

chartView.legend.visible = true
chartView.update()
chartView.grabToImage(function(result) {
    console.log("grabbed")

    var path = filename.toString().replace(/^(file:\/{2})/,"");
    console.log(path + result.saveToFile(path));
    chartView.legend.visible = false
    update();
});

But both those updates happen only after the control comes out of this function, so I don't get legend drawn in PNG. Also I would like the appearance of legend to be unnoticable to user. Are there any ways to do that in QML?

Answer 1

I am unsure, whether I got you quite right.

My suggestion is, to not grab the Item you display, but a copy of it, that you then modify as you like.
Copy here does not mean, that you have the same object twice, but that you render it twice by using a ShaderEffectSource. Before you grab this ShaderEffectSource to image, you can add anything you like.

In my example I display a simple Rectangle with a nice gradient. What I save is the same Rectangle that is extended by the Text 'I am legend'. The user won't see this text apearing in the view at any time.

Rectangle {
    id: commonView
    width: 200
    height: 200
    gradient: Gradient {
        GradientStop { position: 0; color: 'steelblue' }
        GradientStop { position: 1; color: 'orange' }
    }
    MouseArea {
        anchors.fill: parent

        // don't grab the rectangle itself.
        onClicked: legendView.grabToImage(function(result) {
            console.log(result.saveToFile("something.png"));
        });
    }
}

ShaderEffectSource {
    id: legendView
    visible: false // Does not need to be shown.
    sourceItem: commonView
    width: 200
    height: 200
    Text {
        anchors {
            right: parent.right
            bottom: parent.bottom
        }
        text: 'I am legend'
    }
}

You might optimize the performance by only having the ShaderEffectSource active or even created when needed.

You might use the ShaderEffectSource.live-property to disable updating of it. Then use scheduleUpdate() to trigger the update.

This might look like this:

Rectangle {
    id: commonView
    width: 200
    height: 200
    gradient: Gradient {
        GradientStop { position: 0; color: 'steelblue' }
        GradientStop { id: gs1; position: 1; color: 'orange' }
    }
    MouseArea {
        anchors.fill: parent
        onClicked: {
            gs1.position -= 0.1
            legendView.save()
        }
    }
}

ShaderEffectSource {
    id: legendView
    y: 200
    visible: false // Do not render it (will only be rendered when called grapToImage()
    sourceItem: commonView
    width: 200
    height: 200
    live: false // Will only be updated, when explicitly called for
    function save() {
        console.log('Schedule Save')
        scheduleUpdate() // explicitly update. grabToImage() will force rendering afterwards.
        legendView.grabToImage(function(result) {
                        console.log(result.saveToFile("something" +gs1.position.toFixed(1) + ".png"));
                    })
    }

    // Put additional stuff on it.
    Text {
        anchors {
            right: parent.right
            bottom: parent.bottom
        }
        text: 'I am legend!'
    }
}