openscad difference polyhedron

Question

When I try to render this code, it only shows the topmost plane of the trapezoid.

I am trying to cut a cylinder out of this trapezoid, so there is a cylindrical hole in it. I'm using difference to do so. I am pretty sure that I've drawn the trapezoid right. If I remove "difference" from the code, it renders fine. If I take the trapezoid out, the cylinder renders fine (in terrible resolution b/c I've used small numbers.)

Wat

bottomx = 4;
bottomy = 5;
topx = 3;
topy = 5;

difference()
{
    polyhedron (
        points=[ [bottomx,bottomy,0],[bottomx,-bottomy,0],[-bottomx,-bottomy,0],[-bottomx,bottomy,0],
        [topx,topy,5],[topx,-topy,5],[-topx,-topy,5],[-topx,topy,5] ],
    triangles= [ [0,1,2],[2,3,0],
                [4,5,0],[5,1,0],[5,6,1],[6,2,1],[6,7,2],[7,3,2],[7,4,3],[4,0,3],
                [4,5,6],[6,7,4] ]
    );
cylinder([2,2,2], center = true);
}

Answer 1

Your polyhedra isn't actually a valid object. Try compiling with nothing but the polyhedra for testing purposes. If it's invalid it won't show up. I'd guess the triangle order is the problem.

Triangle points ordering-- When looking at the face from the outside inwards, the points must be clockwise. You can rearrange the order of the points or the order they are referenced in each triangle triple. The order of triangles is immaterial. Note that if your polygons are not all oriented the same way OpenSCAD will either print an error or crash completely, so pay attention to the vertex ordering. Again, remember that the 'pN' components of the triangles vector are 0-indexed references to the elements of the points vector.

Also, take a closer look at the error log.

Rendering Polygon Mesh using CGAL...
CGAL error in CGAL_Build_PolySet: CGAL ERROR: assertion violation!
Expr: check_protocoll == 0
File: /usr/include/CGAL/Polyhedron_incremental_builder_3.h