Qt - OpacityMask does not work properly

hi everyone ! i am new in Qt world so, please be nice :D i need to work in .qml, to apply a shader effect over just a part of an image; i tried something like this: i load an image, put a mask (a picture in black and white – white represents the aria where i want to apply the shader); i tried with OpacityMask but it does not work; the example from help does not work either : http://qt-project.org/doc/qt-5.0/qtgraphicaleffects/qml-qtgraphicaleffects1-opacitymask.html#example what am i missing ? any ideas ? thanks