QML opacity

It's easy to change the opacity to child QML elements. It's only setting opacity property between 0(full transparency) to 1(full opacity).

import Qt 4.7
import org.webkit 1.0

Rectangle {
    width: webView.width
    height: webView.height
    Text {
        x: 300
        y: 200
        text: "hidden text"
        smooth: true
    }
    WebView {
        id: webView
        width: 640
        height: 480
        opacity: 0.5
        url: 'http://html5demos.com/canvas-grad'
    }
}

This sample(opacity is 0.5) seems like below:

And if opacity is 0, above sample seems like below: