PyQt4 scroll glitch

Question

I've written a little app, which draws on a QGraphicsView. You can zoom and pan the view. The problem is that when you zoom in (so the view will be bigger than the window), and you pan the view, the flollowing glitch shows(picture)

My code:

import sys
import math
from PyQt4 import QtGui, QtCore



class GraphWidget(QtGui.QGraphicsView):
    def __init__(self):
        super(GraphWidget, self).__init__()
        scene = QtGui.QGraphicsScene(self)
        scene.setItemIndexMethod(QtGui.QGraphicsScene.NoIndex)
        scene.setSceneRect(0, 0, 400, 400)
        self.setScene(scene)
        self.setCacheMode(QtGui.QGraphicsView.CacheBackground)
        self.setViewportUpdateMode(QtGui.QGraphicsView.BoundingRectViewportUpdate)
        self.setRenderHint(QtGui.QPainter.Antialiasing)
        self.setResizeAnchor(QtGui.QGraphicsView.AnchorViewCenter)
        self.sFactor=1
        self.scale(1, 1)
        self.setMinimumSize(400, 400)
        self.setWindowTitle("test")
        self.setDragMode(QtGui.QGraphicsView.ScrollHandDrag)        
        self.setViewportUpdateMode(QtGui.QGraphicsView.FullViewportUpdate)
        #print self.viewportUpdateMode()

    def keyPressEvent(self, event):
        key = event.key()
        if key == QtCore.Qt.Key_Plus:
            self.scaleView(1.2)
        elif key == QtCore.Qt.Key_Minus:
            self.scaleView(1 / 1.2)
        else:
            super(GraphWidget, self).keyPressEvent(event)

    def wheelEvent(self, event):
        self.setTransformationAnchor(QtGui.QGraphicsView.AnchorUnderMouse)
        self.scaleView(math.pow(2.0, -event.delta() / 240.0))

    def drawBackground(self, painter, rect):
        painter.setRenderHint(QtGui.QPainter.Antialiasing, True)
        pen = QtGui.QPen(QtCore.Qt.black, 2/self.sFactor, QtCore.Qt.SolidLine)
        painter.setPen(pen)
        line=painter.drawLine(0, 0, 300, 200)

    def mouseReleaseEvent(self, event):
        #self.update()
        #self.viewport().repaint()
        self.viewport().update()
        #self.scaleView(1.2)
        super(GraphWidget, self).mouseReleaseEvent(event)

    def scaleView(self, scaleFactor):
        self.sFactor = self.matrix().scale(scaleFactor, scaleFactor).mapRect(QtCore.QRectF(0, 0, 1, 1)).width()

        if self.sFactor < 0.07 or self.sFactor > 100:
            return
        self.scale(scaleFactor, scaleFactor)

if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    widget = GraphWidget()
    widget.show()
    sys.exit(app.exec_())

if you call self.scaleView(1.2) in the mousereleaseevent, the glich disappears, but only when the event occurs. I think it's because I don't call the right repaint/update, but I did not find the right one...

Answer 1

Don't change the default Cache Mode for the Background - your issue is solved if you remove this line: self.setCacheMode(QtGui.QGraphicsView.CacheBackground).