Controlling The Pan (to Anchor A Point) When Zooming Into An Image

I'm writing a simple image viewer and am implementing a pan and zoom feature (using mouse dragging and mouse wheel scrolling respectively). I've successfully implemented the pan (e

Solution 1:

Ok, I managed to get it working

defwheelEvent(self, event):
    oldscale = self.scale
    self.scale += event.delta() / 1200.0if (self.scale < 0.1):
        self.scale = oldscale

    screenpoint = self.mapFromGlobal(QtGui.QCursor.pos())
    dx, dy = screenpoint.x(), screenpoint.y()
    oldpoint = (screenpoint.x() + self.position[0], screenpoint.y() + self.position[1])
    newpoint = (oldpoint[0] * (self.scale/oldscale),
                oldpoint[1] * (self.scale/oldscale))
    self.position = (newpoint[0] - dx, newpoint[1] - dy)

the logic here:

• we get the mouses position on the screen (screenpoint), and using this, we have the distance between our anchored pixel and the edge of the screen (by definition)
• we use screenpoint and position to find the coordinate of the mouse in terms of the image's plane (ie: the 2D index of the hovered pixel), as oldpoint
• applying our scaling, we calculate the new 2D index of the pixel (new point)
• we want this pixel on our screen, but not in the top left: we want it dx and dy from the top left (position)

The problem was indeed a trivial confusion between image and display coordinates.