Why does QPainter with underlying OpenGL module make some graphics items so bluray？

[Yanguam Siliagim]

Hi, all! i wanna use OpenGL for handware acceleration, but a phenomenon troubled me. Some items in graphics scene became bluray, and they were very clear before I use OpenGL. What troubled me more, the other items are as clear as before.
Details:

The item in the first picture is painted by Qt native GUI module. It's very clear.

!https://cloud.githubusercontent.com/assets/3531837/3713610/80a3080c-1577-11e4-8c12-77b6d964ffac.jpg(Image1)!

The following pictures show the graphics painted by underlying OpenGL module:

The one below is part of my scene and has the same quality with that painted by native module. It works normally as before.

!https://cloud.githubusercontent.com/assets/3531837/3713609/80a29886-1577-11e4-9860-1acf5103ec6a.jpg(Image2)!

But this one is a terrible blur. This item and the item in Image1 are both instances of the same subclass of QGraphicsObject. Pixmap loaded is as clear as those image1 actually.

!https://cloud.githubusercontent.com/assets/3531837/3713619/1afdee1c-1578-11e4-9454-fa31902f4539.jpg(Image3)!

I used the codes below to set OpenGL as underlying module to render my view:

@
FitView(QGraphicsScene *scene) : QGraphicsView(scene) { //constructor
setSceneRect(Config.Rect); //Config saves user's setting data
setRenderHints(QPainter::TextAntialiasing | QPainter::Antialiasing);

#ifndef QT_NO_OPENGL
if (QGLFormat::hasOpenGL()) {
QGLWidget *widget = new QGLWidget(QGLFormat(QGL::SampleBuffers));

        setViewport(widget);
        setViewportUpdateMode(QGraphicsView::SmartViewportUpdate);
    }

#endif
}
@

Thank you for your concern！