Boolean Union creates Transparent Surface

I ran into this unusual situation so I thought I would share. In the attached file—

Problem Union.3dm (298.9 KB)

—I have three closed polysurfaces:

If I add them together, they produce this transparent surface. If I run the various checks, nothing appears wrong.

It appears that the surfaces are transparent on one side and opaque on the other side.

Hello - yeah, the cylinder surfaces have zero width where the pipe cuts them right at the end caps -

if you rotate the cylinders so the seam is right there, or split them in two there, it should be cleaner.


I should have mentioned that a 90 degree rotation gets rid of the problem. However, it seems like there should be some diagnostic as this would appear to be a bad surface of some kind.