Python 如何在 PyQt5 中裁剪实时视频源 QImage 看起来被剪掉了

我有一个PyQt5图形用户界面应用程序.我从相机中读取一个帧，以在其上 Select 一个较小的区域，稍后我只想显示该区域.我正确地 Select 了该区域，并将其存储在QRect中.但当我试图裁剪要显示的帧并稍后进行处理时，显示的实时视频有时会正常工作.但大多数情况下，它看起来就像这张照片上的那样.它变得倾斜了，右角应该是左角.粉红色的圆圈应该在视频的中间，所以它有点错误.

我有这个Worker1类，我在其中处理显示的帧.

class Worker1(QThread):
    ImageUpdate = pyqtSignal(QImage)

    def __init__(self, canvas, selection):
        super().__init__()
        self.ThreadActive = True
        self.canvas = canvas
        self.anim = None
        self.selection = selection

    def run(self):
        Capture = cv.VideoCapture(0)

        old_closest_contour = None
        d_tresh = 300
        P_color = (0, 255, 0)
        global i
        i = 0

        while self.ThreadActive:
            i += 1
            ret, frame = Capture.read()
            if ret:
                if self.selection is None or self.selection.isEmpty():
                    cropped_frame = frame
                else:
                    a = int(self.selection.x())
                    b = int(self.selection.y())
                    width = int(self.selection.width())
                    height = int(self.selection.height())
                    #print(str(a) + " " + str(b) + " " + str(width) + " " + str(height))
                    cropped_frame = frame[b: b + height, a: a + width].copy()
                    #print(cropped_frame.shape[1])
                    #print(cropped_frame.shape[0])

                ### other processing lines, those are not relevant in the displaying ###

                Image = cv.cvtColor(cropped_frame, cv.COLOR_BGR2RGB)
                img = cv.medianBlur(Image, 25)

                cv.circle(img, (x, y), r, P_color, 1)
                cv.circle(img, (x, y), 4, P_color, -1)
                cv.circle(img, point, 0, (255, 0, 255), 10)

                ConvertToQtFormat = QImage(img.data, img.shape[1], img.shape[0], QImage.Format_RGB888)
                self.ImageUpdate.emit(ConvertToQtFormat)

        Capture.release()