[OpenSCAD] openScad + STL cannot render

cbernhardt charlie at carols62.com
Wed Jan 31 14:57:37 EST 2018


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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