3D CAD model transparency

[Anthonyyg]

I am having difficulty rendering 3D CAD files imported from engineering packages such as Creo, when I render these with V-Ray the refractions are incorrect missing items behind rendering to the environment colour even though the normal pass looks correct and the depth is increased.
I accept that the models are not water tight but is there a way to still use these models or a work around / option within V-Ray material can force correct rendering in these circumstances.
I recall an option within Maya to either simulate or force a solid option, does this exist for 3dsmax? and or could it be implemented.

[kosso_olli]

When working with CAD data, you often have the problem that refractive parts are often modeled without a backside. V-Ray doesn't like this, the refraction is incorrect or it renders black when you use any fog color. I made some tests with Corona a while back, and it handled these situations quite a bit better.

https://forums.chaosgroup.com/forum/...435#post967435

So, make sure your parts have a backside and that the normals are oriented correctly.

[Macker]

Agree with Olli. Try the shell modifier to begin with, or at worst unlink the reflection/refraction IOR and set the refraction IOR to 0.001 or something.

[kosso_olli]

Shell modifier will instantly destroy the normals on imported CAD geometry though. Just to keep in mind.

[Anthonyyg]

Thanks Macker and Olli for your replies. I have checked the CAD it does have back faces but the STL checker confirms that has open edges although the normal pass and edit normal modifier shows that the normals are oriented correctly.
My question is can a V-Ray shader be developed to help work around these issues? The vray clipper has no problem clipping and capping this geometry for example as in this project (second image down).
https://www.behance.net/gallery/3037...-visulisations

[kosso_olli]

The STL checker will always find open edges on imported CAD parts, thats totally normal. It is how NURBS data works, the edges on adjacent surfaces are never joined.
You can also check open edges in the Editable Mesh by the way, no need for STL checker.
As a tip, the open edges can come in handy to display the isolines of the original NURBS surface like this. Much more pleasing to the eye.
Can you share a parts that shows the problem? If it does have backsides, I can't see why it shouldn't refract correctly.