How to set background for qgraphicsscene ?

happyRoll

I used Qt's GrpahicsFrame to made an application of graffiti board. In addition, I want to set background for graphicsscene by QGaphicsScene::drawPixmap(const Qpixmap &pixmap) , it's right, but now this graffiti board is no respond for my mouse, I can't draw some lines on the board. The reason why I want to set a background is hope that I can draw better based on this picture.
if you know any other ways or suggestons, please tell me.

happyRoll

scene = new QGraphicsScene(20, 115, 600, 600,this);
scene->setItemIndexMethod(QGraphicsScene::NoIndex);
canvas = new CanvasItem(this);
scene->addItem(canvas);

graphicsView = new QGraphicsView(this);
graphicsView->setObjectName(QStringLiteral("graphicsView"));
graphicsView->setGeometry(QRect(20, 115, 600, 600));
graphicsView->setMouseTracking(true);
graphicsView->setScene(scene);
QSizePolicy sizePolicy0 (QSizePolicy::Fixed, QSizePolicy::Fixed);
sizePolicy0.setHorizontalStretch(0);
sizePolicy0.setVerticalStretch(0);
graphicsView->setSizePolicy(sizePolicy0);
graphicsView->setFrameStyle(QFrame::Panel | QFrame::Raised);
[QGraphicsView with QPixMap Background](https://forum.qt.io/topic/6313/qgraphicsview-with-qpixmap-background/4)

According this post, I use QgraphicsView::setStyleSheet() to set background, It‘s still useless.
but I comment this statement which "scene->addItem(canvas);", It's work.

//class CanvasItem :public QObject, public QGraphicsItem