How to update QPixmap in QGraphicsView using PyQt

Question

I am trying to draw a QPixmap inside a QGraphicsView. The picture works great, but QGraphicsView does not update it.

Here is the working code:

#!/usr/bin/env python from PyQt4 import QtCore from PyQt4 import QtGui class Canvas(QtGui.QPixmap): """ Canvas for drawing""" def __init__(self, parent=None): QtGui.QPixmap.__init__(self, 64, 64) self.parent = parent self.imH = 64 self.imW = 64 self.fill(QtGui.QColor(0, 255, 255)) self.color = QtGui.QColor(0, 0, 0) def paintEvent(self, point=False): if point: p = QtGui.QPainter(self) p.setPen(QtGui.QPen(self.color, 1, QtCore.Qt.SolidLine)) p.drawPoints(point) def clic(self, mouseX, mouseY): self.paintEvent(QtCore.QPoint(mouseX, mouseY)) class GraphWidget(QtGui.QGraphicsView): """ Display, zoom, pan...""" def __init__(self): QtGui.QGraphicsView.__init__(self) self.im = Canvas(self) self.imH = self.im.height() self.imW = self.im.width() self.zoomN = 1 self.scene = QtGui.QGraphicsScene(self) self.scene.setItemIndexMethod(QtGui.QGraphicsScene.NoIndex) self.scene.setSceneRect(0, 0, self.imW, self.imH) self.scene.addPixmap(self.im) self.setScene(self.scene) self.setTransformationAnchor(QtGui.QGraphicsView.AnchorUnderMouse) self.setResizeAnchor(QtGui.QGraphicsView.AnchorViewCenter) self.setMinimumSize(400, 400) self.setWindowTitle("pix") def mousePressEvent(self, event): if event.buttons() == QtCore.Qt.LeftButton: pos = self.mapToScene(event.pos()) self.im.clic(pos.x(), pos.y()) #~ self.scene.update(0,0,64,64) #~ self.updateScene([QtCore.QRectF(0,0,64,64)]) self.scene.addPixmap(self.im) print('items') print(self.scene.items()) else: return QtGui.QGraphicsView.mousePressEvent(self, event) def wheelEvent(self, event): if event.delta() > 0: self.scaleView(2) elif event.delta() < 0: self.scaleView(0.5) def scaleView(self, factor): n = self.zoomN * factor if n < 1 or n > 16: return self.zoomN = n self.scale(factor, factor) if __name__ == '__main__': import sys app = QtGui.QApplication(sys.argv) widget = GraphWidget() widget.show() sys.exit(app.exec_()) 

mousePressEvent does some painting on QPixmap. But the only solution I found to update the display is to create a new instance (which is not a good solution).

How can I just update it?

Answer 1

The pixel map cannot be attached to your scene, the element uses an internal copy, so you need to update QGraphicsPixmapItem with the new pixmap:

 def __init__(self): ... # save the item as a member self.imItem = self.scene.addPixmap(self.im) ... def mousePressEvent(self, event): if event.buttons() == QtCore.Qt.LeftButton: pos = self.mapToScene(event.pos()) self.im.clic(pos.x(), pos.y()) self.imItem.setPïxmap(self.im) ... 

But it would be wiser to make your Canvas class inherit from QGraphicsPixmapItem instead of QPixmap , you still have to get pixmap with pixmap() , draw on it and call setPixmap to update it. As a bonus, this code will be in the own mousePressEvent element.