UbuntuShape incompatible with Qt Scenegraph renderer batching strategy

The default Qt Scenegraph renderer tries to assemble common primitives in the scenegraph into batches which it can use to reduce the number of draw calls and state changes. See
http://doc-snapshot.qt-project.org/qt5-5.3/qtquick-visualcanvas-scenegraph-renderer.html
for more info.

The UbuntuShape appears to break this batching behaviour.

You can see how Qt selects batches by setting QSG_VISUALIZE=batches - the more unique colors you can see, the less good the batching and the more GL calls Qt needs to make to render your scene.

Here is a file which shows good batching:

import QtQuick 2.0

Item {
    width: 400
    height: 400

    Grid {
        columns: 100
    Repeater {
        model: 1000
        delegate: Image {
                source: "/usr/share/ubuntu-html5-ui-toolkit/0.1/ambiance/img/map_icon.png"
        }
    }
    }
}

If you run that with the QSG_VISUALIZE=batches variable set, you'll see just a block of color on screen - all the delegates were identified to share the same primitives and thus could be placed in one batch. If you run this example under apitrace, you'll notice that the number of GL calls is - once the batch is identified and uploaded, it takes under 50 GL calls to re-render everything. Sure it's a trivial example though.

Now try:

import QtQuick 2.0
import Ubuntu.Components 0.1

Item {
    width: 400
    height: 400

    Grid {
        columns: 100
    Repeater {
        model: 1000
        delegate: UbuntuShape {
            image: Image {
                source: "/usr/share/ubuntu-html5-ui-toolkit/0.1/ambiance/img/map_icon.png"
            }
        }
    }
    }
}

and you'll see every delegate has a different color. This means Qt was unable to recognize that the delegates all had similar basis. If you run under apitrace, you'll see the GPU calls per frame are in the multiple thousands.

We really suffer from this lack of batching in listviews or gridviews where each delegate uses an UbuntuShape.