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I'm trying to model a 3D die with ImplicitRegion[] in order to find out its center of mass. However, when I apply RegionCentroid[] to this region it takes too long to give an output. So I thought DiscretizeRegion[] would might render the computation of the centroid easier without losing precision.

θ=(63/100)Pi;
    depth=9/10;
    r = 22/10;
    offset = 1/2;
    offset61 = 3/10;
    offset62 = 6/10;
    ℛ= ImplicitRegion[
    (*Cube*)
        -1<=x<=1 &&
        -1<=y<=1 &&
        -1<=z<=1 &&
    (*Sphere*)
        x^2+y^2+z^2<=r&&
    (* face 1*)
        Not[(x^2+y^2)(Cos[θ]^2)-((Sin[θ]^2)((z-depth)^2))<=0&&0<(z-depth)<=1] &&
    (* face 2*)
        Not[((x-offset)^2+(z-offset)^2)(Cos[θ]^2)-((Sin[θ]^2)((y+depth)^2))<=0&&-1<(y+depth)<=0]&&
        Not[((x+offset)^2+(z+offset)^2)(Cos[θ]^2)-((Sin[θ]^2)((y+depth)^2))<=0&&-1<(y+depth)<=0]&&
    (*face3*)
        Not[((y-offset)^2+(z-offset)^2)(Cos[θ]^2)-((Sin[θ]^2)((x+depth)^2))<=0&& -1<(x+depth)<=0]&&
        Not[((y)^2+(z)^2)(Cos[θ]^2)-((Sin[θ]^2)((x+depth)^2))<=0&& -1<(x+depth)<=0]&&
        Not[((y+offset)^2+(z+offset)^2)(Cos[θ]^2)-((Sin[θ]^2)((x+depth)^2))<=0&& -1<(x+depth)<=0]&&
    (*face4*)
        Not[((y-offset)^2+(z-offset)^2)(Cos[θ]^2)-((Sin[θ]^2)((x-depth)^2))<=0&& 0<(x-depth)<=1]&&
        Not[((y+offset)^2+(z-offset)^2)(Cos[θ]^2)-((Sin[θ]^2)((x-depth)^2))<=0&& 0<(x-depth)<=1]&&
        Not[((y+offset)^2+(z+offset)^2)(Cos[θ]^2)-((Sin[θ]^2)((x-depth)^2))<=0&& 0<(x-depth)<=1]&&
        Not[((y-offset)^2+(z+offset)^2)(Cos[θ]^2)-((Sin[θ]^2)((x-depth)^2))<=0&& 0<(x-depth)<=1]&&
    (* face 5*)
        Not[((x-offset)^2+(z-offset)^2)(Cos[θ]^2)-((Sin[θ]^2)((y-depth)^2))<=0&&0<(y-depth)<=1]&&
        Not[((x)^2+(z)^2)(Cos[θ]^2)-((Sin[θ]^2)((y-depth)^2))<=0&&0<(y-depth)<=1]&&
        Not[((x-offset)^2+(z+offset)^2)(Cos[θ]^2)-((Sin[θ]^2)((y-depth)^2))<=0&&0<(y-depth)<=1]&&
        Not[((x+offset)^2+(z+offset)^2)(Cos[θ]^2)-((Sin[θ]^2)((y-depth)^2))<=0&&0<(y-depth)<=1]&&
        Not[((x+offset)^2+(z-offset)^2)(Cos[θ]^2)-((Sin[θ]^2)((y-depth)^2))<=0&&0<(y-depth)<=1]&&
    (*face 6*)
        Not[((x+offset62)^2+(y+offset61)^2)(Cos[θ]^2)-((Sin[θ]^2)((z+depth)^2))<=0&&-1<(z+depth)<=0]&&
        Not[((x-offset62)^2+(y+offset61)^2)(Cos[θ]^2)-((Sin[θ]^2)((z+depth)^2))<=0&&-1<(z+depth)<=0]&&
        Not[((x+offset62)^2+(y-offset61)^2)(Cos[θ]^2)-((Sin[θ]^2)((z+depth)^2))<=0&&-1<(z+depth)<=0]&&
        Not[((x-offset62)^2+(y-offset61)^2)(Cos[θ]^2)-((Sin[θ]^2)((z+depth)^2))<=0&&-1<(z+depth)<=0]&&
        Not[((x)^2+(y+offset61)^2)(Cos[θ]^2)-((Sin[θ]^2)((z+depth)^2))<=0&&-1<(z+depth)<=0]&&
        Not[((x)^2+(y-offset61)^2)(Cos[θ]^2)-((Sin[θ]^2)((z+depth)^2))<=0&&-1<(z+depth)<=0]
    , {x,y,z}];
(*b=RegionPlot3D[ℛ,Axes->True, AxesLabel->{x,y,z},PlotStyle->Directive[Opacity[0.9]], PlotPoints->100]*)

When I try to discretize this region I get the following error:

DiscretizeRegion::drf: DiscretizeRegion was unable to discretize the region ImplicitRegion[<<2>>].

I'd like to know:

  1. How to circumvent this error?
  2. Is there a better method to obtain the centroid of this solid?
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1 Answer 1

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You can use the Finite Element mesh generator.

Needs["NDSolve`FEM`"]
mesh = ToElementMesh[\[ScriptCapitalR]]

enter image description here

That already gives a hint to what the problem is because ToElementMesh does not Quiet everything away. Then using the following returns something that does not look terrible:

mesh = ToElementMesh[\[ScriptCapitalR], {{-1.1, 1.1}, {-1.1, 
     1.1}, {-1.1, 1.1}}];
mesh["Wireframe"]

enter image description here

We can convert that to a MeshRegion:

enter image description here

Computing the center

rc = RegionCentroid[MeshRegion[mesh]]
{-0.0007281574436766132`, -0.00209881562008262`, \
0.0035201682587072554`}

Looks like it is slightly off center. I'd play a bit with the boundary resolution to see how much that affects the result.

A visual:

Show[
 mesh["Edgeframe"],
 Graphics3D[{Red, PointSize[0.02], Point[rc]}]
 ]

enter image description here

I did not get very far with a symbolic integration; try numeric integration. You may want to experiment with giving bounds to the ImplicitRegion.

For the statistics buffs: What are my odds for what numbers?

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  • $\begingroup$ The only thing is that ToElementMesh gave me a "unable to discretize ImplicitRegion" error and a very coarse mesh, unlike yours... $\endgroup$
    – Fred
    Nov 28, 2018 at 2:28
  • $\begingroup$ @Fred what version are you on? $\endgroup$
    – user21
    Nov 28, 2018 at 7:01
  • $\begingroup$ Version: 11.3.0.0 (Student Edition); Platform: Mac OS X x86 (32-bit, 64-bit Kernel) $\endgroup$
    – Fred
    Nov 28, 2018 at 14:06
  • $\begingroup$ @Fred, just double checking this is with the bounds specified to ToElementMesh[R,bounds], right. What happens if you use SetSystemOptions[ "FiniteElementOptions" -> {"SymbolicProcessing" -> 10.}]. If that does not help. I'd send this in to support. $\endgroup$
    – user21
    Nov 28, 2018 at 14:15
  • $\begingroup$ I got "FiniteElementOptions" -> {"CacheInterpolationElements" -> True, "DefaultBounds" -> {-1, 1}, "DefaultElementMeshOrder" -> 2, "DefaultExtrapolationHandler" -> {Indeterminate &, "WarningMessage" -> True}, "DefaultNumberOfElements" -> 20, "InterpolationToleranceFactor" -> 0.0001, "MinimumElementMeshQuality" -> 0., "SymbolicProcessing" -> 10.} and reevaluated everything... Still the same. I've included the bounds {{-1.1, 1.1}, {-1.1, 1.1}, {-1.1, 1.1}} $\endgroup$
    – Fred
    Nov 28, 2018 at 14:26

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