Rotate - TransformOriginPoint - PyQt4

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.

Harder

You should edit your post to contain the relevant lines of code where you calculate and apply the transformation (and/or the origin point). Some of the reasons are so this question (if answered) is still useful to others in the distant future (not sure how long pastebin keeps it) and the fact that pastebin is blocked by some company proxies ;) – Standee

Ive edited it. Hope you can help me now :) – Harder

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.

Harder

Answer 2

i am not so familiar with PyQt, but when rotating , most API's rotate around the point ( 0,0 ), so the correct behavior is to : 1 - translate the polygon to 0,0 2- rotate the polygon 3- translate the polygon back

hope that this helps,

regards.

Sollie

Thanks for your suggestion. But doesn't Transform Origin Point , take care of that fact . Nevertheless I will try it soon. – Harder