Graphics3D: Objects with holes

Question

In a recent project, I found out that there is a big limitation when using Graphics3D to construct objects - it lacks the ability to construct objects with holes in them. For example, we can get a regular wall-like Graphics3D object with the Cuboid primitive, but we can not get a wall with a window. Like this one

Naturally, a Boolean operation is the way to go. But, in Mathematica the functions belonging to Boolean operations class, like RegionUnion and RegionIntersection can not be used in Graphics3D

Is there a way to construct Graphics3D objects with holes in them ?

Answer 1

This is a repost of Wolfram Community answer

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Let's start from your code:

cub1 = Cuboid[{0, 0, 0}, {20, 2, 20}];
cub2 = Cuboid[{12, 0, 8}, {17, 2, 17}];
Graphics3D[{cub1, cub2}];
reg = DiscretizeRegion[RegionDifference[cub1, cub2]]

You have a lot of tetrahedrons:

MeshCells[reg, 3] // Length

10093

And a lot of polygons:

MeshCells[reg, 2] // Length  

22096

You do not need all that for visual, - so get the surface (boundary):

surface = BoundaryDiscretizeRegion[reg];
polygons = MeshCells[surface, 2];
polygons // Length

3820

Now so few polygons! Get the coordinates of the mesh too:

points = MeshCoordinates[surface];
points // Length
> 1910

And here you go:

Graphics3D[{EdgeForm[Gray], 
  GraphicsComplex[points, Polygon[polygons /. Polygon[x_] -> x]]}, 
 Boxed -> False, Lighting -> "Neutral"]

Answer 2

Here's one way.

I'm going to use the contourRegionPlot3D function from here. I include the function definition here for convenience:

contourRegionPlot3D[region_, {x_, x0_, x1_}, {y_, y0_, y1_}, {z_, z0_, z1_}, 
  opts : OptionsPattern[]] := Module[{reg, preds},
  reg = LogicalExpand[region && x0 <= x <= x1 && y0 <= y <= y1 && z0 <= z <= z1];
  preds = Union@Cases[reg, _Greater | _GreaterEqual | _Less | _LessEqual, -1];
  Show @ Table[ContourPlot3D[
     Evaluate[Equal @@ p], {x, x0, x1}, {y, y0, y1}, {z, z0, z1}, 
     RegionFunction -> Function @@ {{x, y, z}, Refine[reg, p] && Refine[! reg, ! p]},
     opts], {p, preds}]]

Now create a region from the Graphics3D objects:

c1 = Cuboid[{0, 0, 0}, {20, 2, 20}];
c2 = Cuboid[{12, 0, 8}, {17, 2, 17}];
c3 = Cylinder[{{5, -1, 10}, {5, 2, 10}}, 3];
reg = Fold[RegionDifference, c1, {c2, c3}];

Use RegionMember to turn this into inequalities:

ineqs = Rest @ RegionMember[reg, {x, y, z}]

0 <= x <= 20 && 0 <= y <= 2 && 0 <= z <= 20 && ! (12 <= x <= 17 && 0 <= y <= 2 && 8 <= z <= 17) && ! (0 <= (1 + y)/3 <= 1 && (-5 + x)^2 + (-10 + z)^2 <= 9)

And then:

contourRegionPlot3D[ineqs, {x, 0, 20}, {y, 0, 2}, {z, 0, 20}, 
 Mesh -> None, BoxRatios -> Automatic, PlotRange -> All, Boxed -> False, Axes -> False]