Zoom to the center of view port

I'm currently trying to write a zoomable UserControl and I've currently followed Bob Powell's exam开发者_开发技巧ple at https://web.archive.org/web/20141229192708/http://bobpowell.net/zoompicbox.aspx, 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?

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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)