3D geometry plot in Mathematica

In planar geometry plot question, I asked how to draw planar geometr开发者_开发问答ic constructs. Now I want to extend it to 3D. Not only those geometry packages are not doing well, I am also facing quite a few obstacles in Mathematica.

1. Locator is not usable in 3d, as far as i know.

2. Manipulate does not seem to work in 3d too.

Let me give a concrete example. I have a right circular cone with a height h and an aperture 2 theta. Its circular base is on the horizontal plane. Given a cone element, draw a circle with a diameter d in the tangent plane to this cone passing the cone element. Then draw the horizontal diameter of this circle. Thank you for your help.

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This is really not that hard. First we define a 3D circle, given by a position of its center, and two vectors which span the plane it is in:

Circle3D[{x_, y_, z_}, {v1 : {_, _, _}, v2 : {_, _, _}}, r_] :=
 Line[Table[{x, y, z} + {r Cos[2 Pi t], r Sin[2 Pi t]}.{v1, v2}, {t, 
    0, 1, 1/120}]]

Then given a point {x,y,z} on a cone with tip at {0,0,h} tangents are {x,y,z-h} and {-y,x,0}. The rest is just drawing:

ConeQuestion[h_, theta_, pt : {x_, y_, z_}, 
   d_] /; (x^2 + y^2) Cos[theta]^2 == Sin[theta]^2 (z - h)^2 := 
 Module[{tangents},
  tangents = {Normalize[{0, 0, h} - pt], Normalize[{-y, x, 0}]};
  {{Opacity[0.8, Yellow], Cone[{{0, 0, 0}, {0, 0, h}}, h*Tan[theta]]},
   {Thick, Dashed, Circle3D[pt, tangents, d]},
   {Red, Sphere[pt, 1/10]},
   {Orange, 
    Line[{pt - d Normalize[{-y, x, 0}], 
      pt + d Normalize[{-y, x, 0}]}]}}
  ]