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graphics3d - Dynamic ClipPlanes calculated from current ViewPoint


(This is a follow-up question to the answer found here.)


I would like to define a clip plane dynamically from the current ViewPoint of my Graphics3D. Based on the solution found in the link above, I would like to do something like this (where the ClipPlanes option depends on the ViewPoint variable vp):


DynamicModule[
   {vp},
   {vp} = Options[Graphics3D, ViewPoint][[All, 2]];
   Graphics3D[{FaceForm[Red, Blue], Sphere[]}, Axes -> True, ViewPoint -> Dynamic[vp], 

     ClipPlanes -> {{Sequence@@vp,0}}, ClipPlanesStyle -> Directive[Opacity[.2], Blue]
   ]
]

Unfortunately this doesn't work, and I couldn't figure out how to do it.



Answer



I think this may be the behavior you desire. I am borrowing Kuba's modified ClipPanes specification. One should not need RawBoxes here I think but it does serve the purpose to get our Dynamic expression into the Front End box form.


DynamicModule[{vp},
  {vp} = Options[Graphics3D, ViewPoint][[All, 2]];
  Graphics3D[{FaceForm[Red, Blue], Sphere[]},

    Axes -> True,
    ViewPoint -> Dynamic[vp],
    ClipPlanes -> RawBoxes @ Dynamic @ {{Sequence @@ Cross[vp, {0, 0, 1}], 1}}, 
    ClipPlanesStyle -> Directive[Opacity[.2], Blue]
  ]
]

enter image description here


In Evaluation leak from Dynamic in Button's action John Fultz writes with fair authority:




Front end options, which includes all box options, can take Dynamic heads. That basically means that the FE will compute the value of the Dynamic and use it for the option. And that it will be updated whenever a Dynamic dependency updates.



Since:



  1. Graphic3D expression is transformed into a Graphics3DBox (as may be viewed with menu command Cell > Show Expression)

  2. ClipPlanes remains as an option therein


it stands to reason that this must work if we can keep our Dynamic option value unmolested, and RawBoxes serves to do that.


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