mesh - How to draw the shape of a 2D, 2nd order, element of an ElementMesh

I just want to create a drawing of the shape of 2nd order, two dimensional, finite element like a TriangleElement.

• Something like mesh["Wireframe"] don't work because gives only a first order approximation of the mesh.


• MeshRegion@ToElementMesh[...] don't work because gives some linear boundary instead of curved boundary.



• Something like ToElementMesh@ToBoundaryMesh["Coordinates"->..., "BoundaryElements"->{LineElement[{{1,2,3},...}] don't work because... I don't know :)

I tried this:

Manipulate[Module[{mesh, g},
  mesh = ToElementMesh["Coordinates" -> pts, 
    "MeshElements" -> {TriangleElement[{{1, 3, 5, 2, 4, 6}}]}];
  g = RegionPlot[TrueQ@ElementMeshRegionMember[mesh, {x, y}],
       Evaluate@Prepend[(Mean[#] - {-1.2, 1.2}/2*Subtract @@ # &)@MinMax@pts[[All, 1]], x],
       Evaluate@Prepend[(Mean[#] - {-1.2, 1.2}/2*Subtract @@ # &)@MinMax@pts[[All, 2]], y],
       Prolog -> {

         PointSize[Large], 
         Red, Point@pts[[{1, 3, 5}]],
         Blue, Point@pts[[{2, 4, 6}]]
       },
    PlotPoints -> 40, MaxRecursion -> 1,
    ImageSize -> 300, Frame -> None, PlotRangePadding -> Scaled[.01], 
    Mesh -> Full,
    PlotRange -> Full, PlotRangeClipping -> False, 
    PerformanceGoal -> "Quality"
    ]

  ],
 {{pts, {{0, 0}, {4/7, -(1/39)}, {1, 1/4}, {7/15, 3/10}, {1/4, 2/3}, {-(1/39), 2/5}}}, 
   Locator, LocatorAutoCreate -> False}
 ]

but the result is unsatisfactory and too slow.

I hope to find a fast-enough way accurately represent a complete 2D 2nd order mesh.

Any idea?

Answer

Here is a way to do it. We use BezierCurve for the edges.

First we get the ordering of the egdes. And put the mid side node in the middle.

Needs["NDSolve`FEM`"]
triEdges = #[[{1, 3, 2}]] & /@ 
   MeshElementBaseFaceIncidents[TriangleElement, 2];
quadEdges = #[[{1, 3, 2}]] & /@ 
   MeshElementBaseFaceIncidents[QuadElement, 2];

This function gets us the edges of the elements:

ClearAll[getEdges]

getEdges[ele_TriangleElement] := 
 Join @@ (ElementIncidents[ele][[All, #]] & /@ triEdges)
getEdges[ele_QuadElement] := 
 Join @@ (ElementIncidents[ele][[All, #]] & /@ quadEdges)
getEdges[ele_List] := getEdges /@ ele

The next does the interpolation:

Clear[interpolatingQuadBezierCurve];
interpolatingQuadBezierCurve[pts_List] /; Length[pts] == 3 := 
  BezierCurve[{pts[[1]], 1/2 (-pts[[1]] + 4 pts[[2]] - pts[[3]]), 

    pts[[3]]}];
interpolatingQuadBezierCurve[ptslist_List] := 
  interpolatingQuadBezierCurve /@ ptslist;
interpolatingQuadBezierCurveComplex[coords_, indices_] := 
 interpolatingQuadBezierCurve[Map[coords[[#]] &, indices]]

Try this with an example:

mesh = ToElementMesh[Disk[], "MaxCellMeasure" -> 1, 
   PrecisionGoal -> 1];
Show[

 mesh["Wireframe"["MeshElementStyle" -> EdgeForm[Green]]],
 mesh["Wireframe"["MeshElement" -> "PointElements", 
   "MeshElementIDStyle" -> Blue, 
   "MeshElementStyle" -> Directive[PointSize[0.02], Red]]],
 Graphics[{interpolatingQuadBezierCurveComplex[
      mesh["Coordinates"], #] & /@ 
    Join @@ getEdges[mesh["MeshElements"]]}]
 ]

Looks good:

Green is the linear mesh, black the second order mesh. Next, we try this with your mesh:

mesh = ToElementMesh[
   "Coordinates" -> {{0, 0}, {1, 1/4}, {1/4, 
      2/3}, {4/7, -1/39}, {7/15, 3/10}, {-1/39, 2/5}}, 
   "MeshElements" -> {TriangleElement[{{1, 2, 3, 4, 5, 6}}]}];
Show[
 mesh["Wireframe"["MeshElementStyle" -> EdgeForm[Green]]],
 mesh["Wireframe"["MeshElement" -> "PointElements", 
   "MeshElementIDStyle" -> Blue, 

   "MeshElementStyle" -> Directive[PointSize[0.02], Red]]],
 Graphics[{interpolatingQuadBezierCurveComplex[
      mesh["Coordinates"], #] & /@ 
    Join @@ getEdges[mesh["MeshElements"]]}]
 ]

And here is the Manipulate

Manipulate[
 Module[{mesh}, 

  mesh = ToElementMesh["Coordinates" -> pts, 
    "MeshElements" -> {TriangleElement[{{1, 2, 3, 4, 5, 6}}]}];
  Graphics[
   MapThread[
    interpolatingQuadBezierCurveComplex[mesh["Coordinates"], #] &, 
    getEdges[mesh["MeshElements"]]]
   ]
  ], {{pts, {{0, 0}, {1, 1/4}, {1/4, 2/3}, {4/7, -1/39}, {7/15, 
     3/10}, {-1/39, 2/5}}}, Locator, LocatorAutoCreate -> False}]