Nested Dynamics for moving locators

Question

I'm trying to dynamically draw an ellipse on the image acquired from a camera. See code below:

Dynamic[
 cameraImage = CurrentImage[RasterSize -> {324, 243}];

 Manipulate[
  Show[
   cameraImage
   ,Graphics[{Red, Thick, Circle[pt, Abs[pt2 - pt]*Sqrt[2]]}]
   ],
  {{pt, {324, 243}/2.}, Locator, 
   Appearance -> 
    Style["\[CircleDot]", Red, 30]}, {{pt2, {281, 200}/2.}, Locator, 
   Appearance -> Style["\[FilledCircle]", Red, 16]}
  ]]

I'd like to have the centre of the ellipse independent from the "radius" point (if I move the external point I don't change the position of the centre), while if I move the central point I'd like the ellipse to move consistently (without reshaping), i.e. the external point has to move accordingly to the movement of the centre.

I tried to adapt Kuba's code from here: Moving a locator based on the movement of another but I'm having troubles with nesting dynamic of his code with dynamic in my code.

(I also tried Dr. belisarius answer that kind-of-works but is very laggy and not reliable)

Any suggestion?

Answer 1

For complicated uses of Locator within Manipulate, I usually use a LocatorPane with an explicit Dynamic update function and bypass Manipulate auto-coding.

Manipulate[
 LocatorPane[
  Dynamic[pts, With[{update = #[[1]] - pts[[1]]},
     If[Max@Abs[update] == 0, pts = #, pts += {update, update}]] &],
  Dynamic@Show[
    CurrentImage[RasterSize -> {324, 243}],
    Graphics[{Red, Thick, 
      Circle[pts[[1]], Abs[Subtract @@ pts]*Sqrt[2]]}]],
  Appearance -> {Style["\[CircleDot]", Red, 30], 
    Style["\[FilledCircle]", Red, 16]}
  ],
 {{pts, {{324, 243}/2., {281, 200}/2.}}, ControlType -> None}]