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plotting - RegionPlot3D does not plot "thin" regions?


Basically, I ultimately want to make something like this:


enter image description here


However, I am unsure of how to do this in Mathematica. I'm trying to do this via RegionPlot3D, and for starters, I was trying to draw a little disk in a sphere. But RegionPlot3d does not draws a sphere. For example:


RegionPlot3D[

 x^2 + y^2 + z^2 <= 1  && 
  z > 0 && 
  (x - Cos[Ï€/4] Cos[Ï€/3])^2 + (y - 
       Cos[Ï€/4] Sin[Ï€/3])^2 + (z - Sin[Ï€/4])^2 <= 1/2, 
  {x, -1.5, 1.5}, {y, -1.5, 1.5}, {z, -1.5, 1.5}]

only makes a little blob. If I change this 1/2 to, say, 1/10, then it completely vanishes. Also, if I use x^2+y^2+z^2 == 1 instead of x^2+y^2+z^2 <= 1, nothing is drawn. I tried to cheat and use x^2+y^2+z^2 <= 1 && x^2+y^2+z^2 => 1, but it didn't worked. I don't know what to do.



Answer



Is this what you would like?


SphericalPlot3D[1, {θ, 0, Pi/2}, {ϕ, 0, 2 Pi}, 

 RegionFunction -> 
  Function[{x, y, z, θ, ϕ, r}, 
   z > 0 &&
    (x - Cos[Ï€/4] Cos[Ï€/3])^2 + (y - Cos[Ï€/4] Sin[Ï€/3])^2 + (z - Sin[Ï€/4])^2 <= 1/10],
  PlotRange -> 1.5]

Mathematica graphics



Response to comment:


plot = SphericalPlot3D[1, {θ, 0, Pi}, {ϕ, 0, 2 Pi}, 

 MeshFunctions -> {Function[{x, y, z, θ, ϕ, r},
   (x - Cos[Ï€/4] Cos[Ï€/3])^2 + (y - Cos[Ï€/4] Sin[Ï€/3])^2 + (z - Sin[Ï€/4])^2]}, 
 Mesh -> {{1/10}}, MeshShading -> {Red, Yellow}, 
 BoundaryStyle -> None, PlotPoints -> 50, PlotRange -> 1.5]

Mathematica graphics


The apparent "seam" is the meeting of the boundaries ϕ == 0 and ϕ == 2 π. Since it's a sphere centered at the origin, there is a relatively easy fix. It isn't always so easy.


plot /. GraphicsComplex[pts_, stuff__] :> (GraphicsComplex[pts, stuff] /. 
   (VertexNormals -> _) :> (VertexNormals -> pts))


Mathematica graphics


One can also specify the normals adding the option NormalsFunction.


SphericalPlot3D[..., NormalsFunction -> Function[{x, y, z, θ, ϕ, r}, {x, y, z}]]

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