QGraphicsView additem incorrect size in rezised view

[RDDev]

Hi all,
in my app i'm using a QGraphicsView to display a Microscope image and i want to set a new region for a higher magnification scan. Thus, my program displays an image, enables zoom by mousewheel and the cursor is shaped to a square that shows the user what area he will select. By clicking, i want to set the cursor square to its current position. I created a minimal example of this app with the help of ChatGPT thats shown below.

Currently, the square that is drawn as item and depicted after clicking only coincides for one zooming level to the size of the cursor. Does anyone has a solution how to handle this coordinate mapping correctly?

import sys
from PySide6.QtWidgets import QGraphicsPixmapItem, QApplication, QMainWindow, QGraphicsView, QGraphicsScene, QGraphicsRectItem
from PySide6.QtCore import Qt, QRectF
from PySide6.QtGui import QPixmap, QPainter, QPen, QColor
from PySide6 import QtCore
from PySide6 import QtGui
from PIL import Image, ImageQt
import numpy as np

class GraphicsView(QGraphicsView):
    def __init__(self, scene):
        super().__init__(scene)
        self.setRenderHint(QPainter.Antialiasing)
        self.setMouseTracking(True)
        self.zoom_factor = 1.1
        self.base_cursor_size = 10
        self.rect_item = None
        self.setTransformationAnchor(QGraphicsView.ViewportAnchor.AnchorUnderMouse)
        self.setResizeAnchor(QGraphicsView.ViewportAnchor.AnchorUnderMouse)
        

    def wheelEvent(self, event):
        zoom_in_factor = self.zoom_factor
        zoom_out_factor = 1 / self.zoom_factor

        old_pos = self.mapToScene(event.position().toPoint())

        if event.angleDelta().y() > 0:
            zoom_factor = zoom_in_factor
        else:
            zoom_factor = zoom_out_factor

        self.scale(zoom_factor, zoom_factor)

        new_pos = self.mapToScene(event.position().toPoint())
        delta = new_pos - old_pos
        self.translate(delta.x(), delta.y())

        self.set_square_cursor()

    def mousePressEvent(self, event):
        if event.button() == Qt.LeftButton:
            if self.rect_item:
                self.scene().removeItem(self.rect_item)

            scene_pos = self.mapToScene(event.position().toPoint())
            rect_size = 10 / self.transform().m11()  # Adjust rectangle size according to zoom level
            self.rect_item = QGraphicsRectItem(scene_pos.x()-rect_size/2, scene_pos.y()-rect_size/2, rect_size, rect_size)
            self.rect_item.setPen(QPen(QColor(0, 127, 255), 1 / self.transform().m11()))
            self.scene().addItem(self.rect_item)
        super().mousePressEvent(event)


    def enterEvent(self, event: QtCore.QEvent) -> None:
        self.set_square_cursor()
        super().enterEvent(event)

    def leaveEvent(self, event: QtCore.QEvent) -> None:
        self.setCursor(QtCore.Qt.CursorShape.ArrowCursor)
        super().leaveEvent(event)

    def set_square_cursor(self):
        # Calculate the cursor size based on the current zoom level
        transform = self.transform()
        scale_factor = transform.m11()
        cursor_size = int(self.base_cursor_size * scale_factor)

        cursor_arr = np.zeros((cursor_size, cursor_size), dtype=np.uint8)
        padded_cursor_arr = np.pad(cursor_arr, 1, constant_values=1)
        rgba_cursor = np.stack([padded_cursor_arr] * 4, axis=-1) * [[[255, 127, 0, 255]]]
        rgba_cursor = rgba_cursor.astype(np.uint8)

        qim = QtGui.QImage(rgba_cursor, rgba_cursor.shape[1], rgba_cursor.shape[0], QtGui.QImage.Format.Format_ARGB32)
        pix = QtGui.QPixmap.fromImage(qim)
        cursor = QtGui.QCursor(pix, -cursor_size // 2, -cursor_size // 2)
        self.setCursor(cursor)

class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        self.setWindowTitle("Zoomable QGraphicsView with Fixed Rectangle Cursor")
        self.setGeometry(100, 100, 800, 600)

        self.scene = QGraphicsScene()
        self.view = GraphicsView(self.scene)
        self.setCentralWidget(self.view)

        # add example image
        self.add_example_image()

    def add_example_image(self):
        rect = QRectF(0, 0, 800, 600)
        self.scene.setSceneRect(rect)

        # create example image
        pixmap = QPixmap.fromImage(ImageQt.ImageQt(Image.fromarray(np.random.randint(0, 20, (300, 400, 3), dtype=np.uint8))))
        pixmap_item = QGraphicsPixmapItem(pixmap)
        scale = 500 / pixmap_item.pixmap().width()
        pixmap_item.setScale(scale)
        pixmap_item.setPos(0, 0)
        self.scene.addItem(pixmap_item)

def main():
    app = QApplication(sys.argv)
    window = MainWindow()
    window.show()
    sys.exit(app.exec())

if __name__ == "__main__":
    main()

Best and thanks in advance,
RDDev

[SGaist]

Hi and welcome to devnet,

Shouldn't:
rect_size = 10 / self.transform().m11() # Adjust rectangle size according to zoom level
rather be:
rect_size = 10 * self.transform().m11() # Adjust rectangle size according to zoom level

?