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From this question I'm using a function by kglr to find the inequality constraints for a shape.

The function is this 

constraints[shapes__] := 
 And[## & @@ (Not /@ 
      Through[(RegionMember[RegionIntersection@##] & @@@ 
          Subsets[{shapes}, {2}])@#]), 
   RegionMember[RegionUnion @@ (RegionBoundary /@ {shapes})]@#] &

This works great if I have a shape with just integers. The inequalities is exactly what I would expect 

constraints[Cuboid[{1, 1, 1}, {2, 2, 2}]][{x, y, z}]

Returns

(x | y | z) \[Element] Reals && ((-1 + z == 0 && -1 + y >= 0 && 3 - x - y >= 0 &&  2 - x <= 1) || (-1 + z == 0 &&   2 - y >= 0 && -3 + x + y >= 0 && -1 + x <= 1) || (-1 + x == 0 && -1 + y >= 0 && -y + z >= 0 && -1 + z <= 1) || (-1 + x == 0 && 2 - y >= 0 && y - z >= 0 && 2 - z <= 1) || (-2 + y == 0 && -1 + x >= 0 && -x + z >=  && -1 + z <= 1) || (-2 + y == 0 && 2 - x >= 0 && x - z >= 0 &&  2 - z <= 1) || (-2 + x == 0 && 2 - y >= 0 && -3 + y + z >= 0 && -1 + z <= 1) || (-2 + x == 0 && -1 + y >= 0 && 3 - y - z >= 0 &&  2 - z <= 1) || (-1 + y == 0 &&  2 - x >= 0 && -3 + x + z >= 0 && -1 + z <= 1) || (-1 + y == 0 && -1 + x >= 0 && 3 - x - z >= 0 &&   2 - z <= 1) || (-2 + z == 0 && -1 + y >= 0 && x - y >= 0 && -1 + x <= 1) || (-2 + z == 0 && 2 - y >= 0 && -x + y >= 0 && 2 - x <= 1))

Which is great. However, if within the shape definition there a non-integer value, the function will not return an analytical result, i.e. 

 constraints[Cuboid[{1, 1, 1}, {2, 2, 1.5}]][{x, y, z}]

will not work. Note, that the second vector in Cuboid has a 1.5. I cannot find out what section of the function is causing the problem & how I might solve it. At the moment I manually go through and amend the inequalities - this is not ideal.

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    $\begingroup$ As a workaround, you may use rational numbers: constraints[Cuboid[{1, 1, 1}, {2, 2, 3/2}]][{x, y, z}] works fine. Apparently, RegionMember assumes that there is no reasonable chance to return a analytic function if there are floating point number present (which is, well, wrong). But it is actually very desirable that RegionMember is overly conservative: This way it does not waste precious time building the function in case of too complicated expression. $\endgroup$
    – Henrik Schumacher
    Feb 25, 2018 at 20:42

1 Answer 1

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What happens is that with approximate numbers (e.g. 1.5), approximate methods are invoked, namely in this case, the region is discretized:

First@Head@constraints[Cuboid[{1, 1, 1}, {2, 2, 1.5}]][{x, y, z}]

Mathematica graphics

% // InputForm
(*
MeshRegion[
 {{2., 1., 1.}, {2., 2., 1.}, {1., 2., 1.}, {1., 1., 1.}, {1., 2., 1.5},
  {1., 1., 1.5}, {2., 2., 1.5}, {2., 1., 1.5}},
 {Polygon[{{1, 3, 4}, {4, 5, 6}, {3, 7, 5}, {2, 8, 7}, {1, 6, 8}, {6, 7, 8},
   {3, 1, 2}, {5, 4, 3}, {7, 3, 2}, {8, 2, 1}, {6, 1, 4}, {7, 6, 5}}]}]
*)

Numerical methods are used to compute the RegionMemberFunction for a MeshRegion, so a symbolic formula is not produced.

With exact input, as Henrik suggested, exact methods are used and if feasible, an exact symbolic description of the region will be produced.

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