Weird result when combine DirectionalBlur and zoomBlur(QtGraphicalEffect)

[stereomatching]

compiler : clang3.2
os : mac osx 10.8.3
Qt ver : 5.1Beta

EX 1 :
@
Rectangle{
id : root
width : 1024
height : 768

state: "NORMAL"

Image{
    id: img

    anchors.fill: root
    smooth: true
    fillMode: Image.PreserveAspectFit
    source: "Downloads/lena.tif"

    DirectionalBlur {
        anchors.fill: img
        source: img
        angle: 90
        length: 32
        samples: 24
    }

    /*ZoomBlur {
        anchors.fill: img
        source: img
        samples: 0
        length: 0
    }*/
}

}
@

"The results ":http://www.flickr.com/photos/92283971@N04/8930859400/

@
Rectangle{
id : root
width : 1024
height : 768

state: "NORMAL"

Image{
    id: img

    anchors.fill: root
    smooth: true
    fillMode: Image.PreserveAspectFit
    source: "Downloads/lena.tif"

    DirectionalBlur {
        anchors.fill: img
        source: img
        angle: 90
        length: 32
        samples: 24
    }

    ZoomBlur {
        anchors.fill: img
        source: img
        samples: 0
        length: 0
    }
}

}
@

"The results ":http://www.flickr.com/photos/92283971@N04/8930260131/

Since the samples and length of ZoomBlur are zero, it should not have any effect to the image
Infact, combination of RadialBlur and DirectionalBlur would bring weird result too, but combination of
RadialBlur and ZoomBlur have no problem(atleast my eye can't tell it)