Question

I was trying to build a small UI using PyQt .

It has a window , a button(Rotate) , and a polygon(rectangle) in a QGraphicsView. One aim of
the app is to allow the user to rotate the polygon. That is after the
button is clicked and the user clicks at a point , the nearest vertex
automatically shifts or tilts towards the user click. Ive also set the
polygon to be movable before the click and not movable after the click.

The problem is if the user moves the polygon and then clicks , the polygon
rotates in a weird manner. Could someone help me figure out the error? My guess is that it might be with the setTransformOriginPoint.

EDIT:
I have two classes inherited from QtGui.QWidget and QtGui.QGraphicsScene.

class Window(QtGui.QWidget):
    def polychange(self , sender): //Called by the button , 
        if sender:
                self.view.polyrotate = 1 //self.view is an instance of QGraphicsScene class
                self.view.polyf.setFlag(QtGui.QGraphicsItem.ItemIsMovable , False)

        else:
                self.view.polyrotate = 0 
                self.view.degrees = 0
                self.view.polyf.setFlag(QtGui.QGraphicsItem.ItemIsMovable)

class Example(QtGui.QGraphicsView):
    def mousePressEvent(self , e):
        super(Example , self).mousePressEvent(e)
        self.x = e.x()
        self.y = e.y()
        if self.polyrotate == 1:
            self.Rotate()

    def Rotate(self):

        self.pverticesx = []
        self.pverticesy = []
        distances = []
        for i in range(4):
            self.pverticesx.append(self.polyf.mapToScene(self.polyf.polygon()[i]).x())
            self.pverticesy.append(self.polyf.mapToScene(self.polyf.polygon()[i]).y())

        x1 = self.x
        y1 = self.y      

        for i in range(4):
             distance = math.hypot(self.pverticesx[i] - x1 , self.pverticesy[i] - y1)
             distances.append(distance)
        midpointx = (self.pverticesx[0] +  self.pverticesx[2]) / 2
        midpointy = (self.pverticesy[0] +  self.pverticesy[2]) / 2
        index = distances.index(min(distances))          
        pointx = self.pverticesx[index]                
        pointy = self.pverticesy[index]
        vector1 = [x1 - midpointx , y1 - midpointy]
        vector2 = [pointx - midpointx , pointy - midpointy]
        num = 0

        for i in [0 , 1]:
            num = num + (vector1[i] * vector2[i])  
        den = math.sqrt(sum(map(lambda x : x * x , vector1))) *  math.sqrt(sum(map(lambda x : x * x , vector2)))

        degree = math.degrees(math.acos(num / den))
        self.degrees = degree + self.degrees
        if self.degrees > 360:
            rotation = self.degrees / 360
            self.degrees = self.degrees - (rotation * 360)

        self.polyf.setTransformOriginPoint(midpointx , midpointy)
        self.polyf.setRotation(self.degrees)

Here is a more exhaustive link to my code. RotateApp . Thanks in advance.

Answer 1

I’ve found out the answer to my own question. Before rotating , midpointx and midpointy are in scene coordinates. So I had to map them back to QGraphicsItem coordinates system. This line before self.setRotation() would do the trick.

self.polyf.setTransformOriginPoint(self.polyf.mapFromScene(QtCore.QPointF(midpointx , midpointy)))

Now my polygon doesnt move randomnly.