[OpenSCAD] openScad + STL cannot render

Chris Camacho chris at bedroomcoders.co.uk
Wed Jan 31 15:03:49 EST 2018

I don't know about the likes of AutoCAD as I'm too tight to buy 
propriety software and comfortable with my tin foil hat.....

however in blender you can "recalculate outside", "recalculate inside" 
and also flip normals

You can also display normals which I usually do, just as a quick check...

On 31/01/18 19:57, cbernhardt wrote:
> I believe the problem might be caused by the normal vectors in the STL file.
> To test this I drew a 100 x 100 x 100 cube in AutoCAD and randomly drew
> 3DFACES (three dimensional triangular facets) to cover each side, 12
> triangles total.  I then exported the cube as an STL file, calculating the
> normal vector of each triangle as the cross product of the first two sides
> as drawn.
> When I imported the cube.stl  file into OpenSCAD it previewed (F5) and
> rendered (F6) correctly but when I added an OpenSCAD object (i.e. an
> extruded polygon) both objects previewed correctly but rendering (F6) caused
> the STL object to disappear.
> I wrote a routine inside AutoCAD to display the normal vectors when
> importing an STL file and discovered that some of them, depending on the
> direction in which the triangle had been drawn, did not point to the outside
> of the cube.  After redrawing the suspect triangles so that all normal
> vectors pointed to the outside of the cube, everything worked correctly in
> OpenSCAD.
> I don't know if there is a program to automatically "fix" the normal
> vectors.  Perhaps some of the more experienced OpenSCAD users know of such a
> program??
> Charles
> --
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