How to get the image buffer of videoOutput?

[stereomatching]

@
Rectangle {
width: 320
height: 240

Camera{
    id: camera
}

VideoOutput {
    id: videoOutput

    anchors.fill: parent
    source: camera
}    

GammaAdjust {
    id: gammaAdjust

    anchors.fill: videoOutput
    source: videoOutput
    gamma: 0.5
}

}
@

The source of the videoOutput come from camera
then process by the GammaAdjust
How could I get the image after process?
Any workaround?