Why does the union of a primitive and a foreign STL makes a mesh that is not closed?

Question

I am somewhat familiar with openscad but am having some trouble with this.

I have found an STL file on the web.

I use this code:

translate([-11.5,-22,0])
    scale([1.2,1.2,0.8])
        rotate([-72,-10,-2])
            import("/home/john/Downloads/Baby_Groot_Head_Keychain.stl");

scale([0.75,1.2,1])
    cylinder(r=25,h=4);

Here's a pic if it helps:

But when it renders, I see:

Parsing design (AST generation)...
Saved backup file: /home/john/.local/share/OpenSCAD/backups/unsaved-backup-ngqIJBgz.scad
Compiling design (CSG Tree generation)...
Rendering Polygon Mesh using CGAL...
ERROR: The given mesh is not closed! Unable to convert to CGAL_Nef_Polyhedron. 
Geometries in cache: 12
Geometry cache size in bytes: 3492096
CGAL Polyhedrons in cache: 4
CGAL cache size in bytes: 324928
Total rendering time: 0:00:00.157
   Top level object is a 3D object:
   Simple:        yes
   Vertices:       60
   Halfedges:     180
   Edges:          90
   Halffacets:     64
   Facets:         32
   Volumes:         2
Rendering finished.

Do you have any tips to resolve this?

Answer 1

I had the same issue (difference of a foreign STL and a cube() causing the ERROR: The given mesh is not closed! Unable to convert to CGAL_Nef_Polyhedron. error). I think that OpenSCAD needs to know the convexity of the imported STL in order to compute intersections / unions accurately without leaving open faces, the problem being that the import function has a default value of 3 for convexity.

In my case, increasing this parameter a bit fixed the issue. In the hair of your Groot model, it looks like there may be a few ridges that make the convexity of the model a bit high and causing issues. I count a few dozen, so let's try with convexity=30 and see if the issue is fixed. If this fixes the issue but makes renders slow, you can try to reduce the parameter in small steps.

Your code would look like this:

translate([-11.5,-22,0])
    scale([1.2,1.2,0.8])
        rotate([-72,-10,-2])
            import("/home/john/Downloads/Baby_Groot_Head_Keychain.stl", convexity=30);

scale([0.75,1.2,1])
    cylinder(r=25,h=4);