how to get only image pixel from QGraphicsScene - opencv

Question

here is my simple code to display the image in QGraphicsView in pyqt python 3.7. I want an image pixel when the mouse is pressed on a scene or window of QGraphicsView or QGraphicsScene.
Mouse Press Function
Mouse Press Event Handler
def mousePressEvent(self):
p = QtGui.QCursor.pos()
print("pressed here: ", p)
Mouse Press Event caller
self.scene1.mousePressEvent = mousePressEvent
Main Code
import cv2
from PyQt5 import QtCore, QtGui, QtWidgets
from PyQt5.QtGui import *
from PyQt5.QtWidgets import QGraphicsScene, QAction
class Ui_MainWindow(object):
def setupUi(self, MainWindow):
MainWindow.setObjectName("MainWindow")
MainWindow.resize(800, 600)
self.centralwidget = QtWidgets.QWidget(MainWindow)
self.centralwidget.setObjectName("centralwidget")
self.graphicsView = QtWidgets.QGraphicsView(self.centralwidget)
self.graphicsView.setGeometry(QtCore.QRect(20, 10, 761, 561))
self.graphicsView.setObjectName("graphicsView")
MainWindow.setCentralWidget(self.centralwidget)
self.statusbar = QtWidgets.QStatusBar(MainWindow)
self.statusbar.setObjectName("statusbar")
MainWindow.setStatusBar(self.statusbar)
self.retranslateUi(MainWindow)
QtCore.QMetaObject.connectSlotsByName(MainWindow)
# ---- Mouse Press Event Handler ---- #
def mousePressEvent(self):
p = QtGui.QCursor.pos() # Here I want image pixel coordinate (x,y) how we can..?
print("pressed here: ", p)
# ---- Mouse Press Event caller ---- #
self.scene1.mousePressEvent = mousePressEvent
def retranslateUi(self, MainWindow):
_translate = QtCore.QCoreApplication.translate
MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
# -------------------------------------------------
image = cv2.imread('lena.jpg') # Read image
image = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
height, width = image.shape # read image size
self.image_disp = QImage(image.data, width, height, QImage.Format_Grayscale8)
# -------------------------------------------------
self.scene1 = QGraphicsScene()
pixMap = QPixmap.fromImage(self.image_disp)
self.scene1.addPixmap(pixMap)
self.graphicsView.setScene(self.scene1)
if __name__ == "__main__":
import sys
app = QtWidgets.QApplication(sys.argv)
MainWindow = QtWidgets.QMainWindow()
ui = Ui_MainWindow()
ui.setupUi(MainWindow)
MainWindow.show()
sys.exit(app.exec_())

Answer 1

You should not modify the class generated by Qt Designer(1), instead create another class that inherits from a widget and use the initial class as an interface.
Do not override the mousePressEvent method using self.scene1.mousePressEvent = mousePressEvent because you are deleting the default implementation, instead you can create a class that inherits from QGraphicsScene or use an event filter, in this case I will use the second method.
To obtain the position of the mouse with respect to the image (QGraphicsPixmapItem), you must use the transformations between the different elements of the Qt Graphics Framework.
import os
import cv2
from PyQt5 import QtCore, QtGui, QtWidgets
class Ui_MainWindow(object):
def setupUi(self, MainWindow):
MainWindow.setObjectName("MainWindow")
MainWindow.resize(800, 600)
self.centralwidget = QtWidgets.QWidget(MainWindow)
self.centralwidget.setObjectName("centralwidget")
self.graphicsView = QtWidgets.QGraphicsView(self.centralwidget)
self.graphicsView.setGeometry(QtCore.QRect(20, 10, 761, 561))
self.graphicsView.setObjectName("graphicsView")
MainWindow.setCentralWidget(self.centralwidget)
self.statusbar = QtWidgets.QStatusBar(MainWindow)
self.statusbar.setObjectName("statusbar")
MainWindow.setStatusBar(self.statusbar)
self.retranslateUi(MainWindow)
QtCore.QMetaObject.connectSlotsByName(MainWindow)
def retranslateUi(self, MainWindow):
_translate = QtCore.QCoreApplication.translate
MainWindow.setWindowTitle(_translate("MainWindow", "MainWindow"))
class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
def __init__(self, parent=None):
super().__init__(parent)
self.setupUi(self)
self.scene = QtWidgets.QGraphicsScene(self)
self.graphicsView.setScene(self.scene)
self.scene.installEventFilter(self)
current_dir = os.path.dirname(os.path.realpath(__file__))
filename = os.path.join(current_dir, "lena.jpg")
image = cv2.imread(filename)
image = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
height, width = image.shape
image_disp = QtGui.QImage(
image.data, width, height, QtGui.QImage.Format_Grayscale8
)
pixMap = QtGui.QPixmap.fromImage(image_disp)
self.pixmap_item = self.scene.addPixmap(pixMap)
def eventFilter(self, obj, event):
if obj is self.scene and event.type() == QtCore.QEvent.GraphicsSceneMousePress:
spf = event.scenePos()
lpf = self.pixmap_item.mapFromScene(spf)
brf = self.pixmap_item.boundingRect()
if brf.contains(lpf):
lp = lpf.toPoint()
print(lp)
return super().eventFilter(obj, event)
if __name__ == "__main__":
import sys
app = QtWidgets.QApplication(sys.argv)
w = MainWindow()
w.show()
sys.exit(app.exec_())
(1) Using the Generated Code