Pyqt4: How to correct QGraphicsItem position? - position

Question

I'm trying to draw a path on a QGraphicsView. However, the position seems not right. The first point(red) is (0,0), which is supposed to be at the top-left corner. How do I move the drawing to the right position?
from PyQt4 import QtGui, QtCore
from PyQt4.QtCore import QPointF as qpf
import sys
from PyQt4.QtGui import QPainterPath
data= [qpf(0,0),qpf(40,30),qpf(30,60),qpf(70,90),qpf(20,120),qpf(60,150)]
class Window(QtGui.QWidget):
def __init__(self):
QtGui.QWidget.__init__(self)
self.view = View(self)
layout = QtGui.QVBoxLayout(self)
layout.addWidget(self.view)
class View(QtGui.QGraphicsView):
def __init__(self, parent):
QtGui.QGraphicsView.__init__(self, parent)
self.setScene(QtGui.QGraphicsScene(self))
item = QtGui.QGraphicsRectItem(data[0].x()-2,data[0].y()-2,4,4)
item.setBrush(QtCore.Qt.red)
self.scene().addItem(item)
self.path = path = QPainterPath(data[0])
for d in data[1:]:
path.lineTo(d)
item = QtGui.QGraphicsPathItem(path)
self.scene().addItem(item)
def mouseReleaseEvent(self, event):
pos = event.pos()
rect = QtCore.QRectF(pos.x()-2, pos.y()-2,4,4)
item = QtGui.QGraphicsRectItem(rect)
self.scene().addItem(item)
if self.path.intersects(rect):
print 'on line'
else:
print 'Not belong to line (%d, %d)' % (pos.x(), pos.y())
QtGui.QGraphicsView.mouseReleaseEvent(self, event)
if __name__ == '__main__':
app = QtGui.QApplication(sys.argv)
window = Window()
window.resize(400, 400)
window.show()
sys.exit(app.exec_())

Answer 1

This behavior is because you draw using QGraphicsScene and QGraphicsView. This is because view and scene have some automatism, which are very convenient, normally. The scene coordinated can be completely different from the view (which is in pixels).
From the docs: "After you call show(), the view will by default scroll to the center of the scene and display any items that are visible at this point." show() is called implicitly in your case, I believe after you add an item to the scene.
I can think of two possibilities to get what you want:
1) change the view onto your scene, so that the scene-coordinate (0, 0) is in the upper left corner of your view.
2) Do NOT use QGraphicsScene and QGraphicsView, but just draw on a widget from its paint event as shown for example here. This means your dimensions are all in pixels, i.e. the coordinates of your points are pixels. And no automatism that might confuse you is done.

Answer 2

You would want to use self.setAlignment(QtCore.Qt.AlignLeft|QtCore.Qt.AlignTop) in your view class constructor, that will make 0,0 of the scene coordinates the same as 0,0 in view coordinates.AlignmentFlag
I would also suggest pos = self.mapToScene(event.pos()) in the mouseReleaseEvent. that way if you zoom in or scroll the scene, the click will happen in the right place in the view. QGraphicsView.mapFromScene