Issue with not pre-rendering difference

HullBetweenIssue.scad

DETAIL=120;

module hullBetween()
{
    // Combine the hull between and the original objects
    difference()
    {
        hull()
        {
            children();
        }

        for (childIndex = [0 : $children-1] )
        {
            difference()
            {
                hull()
                {
                    children(childIndex);
                }
                children(childIndex);
            }
        }
    }
}

module testHullBetweenShape()
{
    render()
    difference()
    {
        cylinder(5, 20, 20, $fn=DETAIL);

        translate([0,0,-1])
        cylinder(6, 17.5, 17.5, $fn=DETAIL);
        translate([16,0,-1])
        cube([10,10,12],center=true);
    }
}

module testHullBetween()
{
    hullBetween()
    {
        testHullBetweenShape();
        
        translate([-100,-100,0])
        rotate([0,0,-90])
        testHullBetweenShape();

        translate([-10,-100,0])
        rotate([0,0,-90])
        testHullBetweenShape();
    }
}

testHullBetween();

It is not too surprising that when you subtract an object from its own hull you get lots of thin slivers. I suspect this is due to hull and difference using different number representations.

Inflating the object by a small amount before subtracting it from its hull by using minkowski with a sphere fixes it but it very slow of course.

Right, I'm not surprised by that. I'm surprised that the render() on on line 30 fixes the issue in all the cases that I have. I tried adding render() all over the place in hullBetween() to no effect.

Note also that without the render(), there are artifacts far outside of the boundary of the hull of the object that make the resulting STL non-manifold. That was the even more frustrating part of this.