Square tube with hole renders incorrectly

[nophead]

This is a square tube with a hole drilled through it.

difference() {
render() rotate([90, 0, 0])
linear_extrude(height = 100, convexity = 10, center = true)
difference() {
square([15, 15], center = true);
square([10, 10], center = true);
}
cylinder(r = 5/2, h = 20, center = true);
}

It renders with missing faces with F5, OK with F6.

Removing the render() statement, or moving it outside the difference() fixes it.

[kintel]

This looks like the same issue as #112. Does that sounds right to you?

[nophead]

No I don't think so. The linear_extrude on its own works despite having a square hole in the middle of a square. It is drilling a hole through the resultant square tube that fails but only when the tube has been render()ed but the hole hasn't.

[kintel]

render(convexity=2) should do the trick.

convexity is such a confusing concept..

If you agree, please close this issue. Fixing convexity is kind of not on the radar yet ;)

[nophead]

Yes that fixes it. I hadn't realised that render() had a convexity parameter and have got away without ever setting it before, despite there being a render around every vitamin and stl in Mendel90.

The confusing thing about convexity is the thing being rendered or extruded can appear fine on its own but goes wrong when you do some later operation on it.