How to get cursor click position in QGraphicsItem coordinate system?

Question

I have a QGraphicsScene with QGraphicsItem added to it. Suppose I clicked on a map image (QGraphicsItem) where green circle is drawn. How to get click position in terms of this QGraphicsItem and not QGraphicsScene coordinate system.

P.S. Please, don't write code with mouse event handling. Just how to map click position correctly. Thanks in advance.

Answer 1

The idea is to convert the coordinate with respect to the scene to a coordinate with respect to the item.

- Override mousePressEvent in QGraphicsScene:

using the mapFromScene() method of QGraphicsItem:

from PyQt5 import QtCore, QtGui, QtWidgets
import random

class Scene(QtWidgets.QGraphicsScene):
    def __init__(self, parent=None):
        super(Scene, self).__init__(parent)
        pixmap = QtGui.QPixmap(100, 100)
        pixmap.fill(QtCore.Qt.red)

        self.pixmap_item = self.addPixmap(pixmap)
        # random position
        self.pixmap_item.setPos(*random.sample(range(-100, 100), 2))


    def mousePressEvent(self, event):
        items = self.items(event.scenePos())
        for item in items:
            if item is self.pixmap_item:
                print(item.mapFromScene(event.scenePos()))
        super(Scene, self).mousePressEvent(event)

if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    scene = Scene()
    w = QtWidgets.QGraphicsView(scene)
    w.resize(640, 480)
    w.show()
    sys.exit(app.exec_())

- Override mousePressEvent in QGraphicsView:

using the mapFromScene() method of QGraphicsItem with mapToScene():

from PyQt5 import QtCore, QtGui, QtWidgets
import random

class View(QtWidgets.QGraphicsView):
    def __init__(self, parent=None):
        super(View, self).__init__(QtWidgets.QGraphicsScene(), parent)
        pixmap = QtGui.QPixmap(100, 100)
        pixmap.fill(QtCore.Qt.red)

        self.pixmap_item = self.scene().addPixmap(pixmap)
        # random position
        self.pixmap_item.setPos(*random.sample(range(-100, 100), 2))


    def mousePressEvent(self, event):
        items = self.items(event.pos())
        for item in items:
            if item is self.pixmap_item:
                print(item.mapFromScene(self.mapToScene(event.pos())))
        super(View, self).mousePressEvent(event)

if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = View()
    w.resize(640, 480)
    w.show()
    sys.exit(app.exec_())

- Override mousePressEvent of QGraphicsItem:

from PyQt5 import QtCore, QtGui, QtWidgets
import random

class PixmapItem(QtWidgets.QGraphicsPixmapItem):
    def mousePressEvent(self, event):
        print(event.pos())
        super(PixmapItem, self).mousePressEvent(event)

if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    scene = QtWidgets.QGraphicsScene()
    w = QtWidgets.QGraphicsView(scene)
    pixmap = QtGui.QPixmap(100, 100)
    pixmap.fill(QtCore.Qt.red)
    item = PixmapItem(pixmap)
    scene.addItem(item)
    item.setPos(*random.sample(range(-100, 100), 2))
    w.resize(640, 480)
    w.show()
    sys.exit(app.exec_())