Zoom to the center of view port

Question

I'm currently trying to write a zoomable UserControl and I've currently followed Bob Powell's example at http://www.bobpowell.net/zoompicbox.htm, however, what I'm trying to achieve is zooming to the center point of the control not the top left corner. Is anybody having experience/idea on this? thanks.

Answer 1

You need to offset the values of the control by the centre, apply the scale transform and then offset back.

Try the following algorithm to calculate the new top left:

(-old_centre * factor) + old_centre

If the control is 100,100 a zoom factor of 2 will produce:

((-50, -50) * 2) + (50, 50) = (-100, -100) + (50, 50) = (-50, -50)

With a zoom factor of 0.5:

((-50, -50) * 0.5) + (50, 50) = (-25, -25) + (50, 50) = (25, 25)