How to preserve smoothing groups while doing Edit Poly attach?


I brought some CAD geometry into 3ds Max 2017. I’m attaching objects that share the same materials (optimizing for use in a game engine). The geometry has smoothing groups and looks pretty good but when I try to attach with either Edit Poly or Edit Mesh, the smoothing groups are lost and the geometry looks like crap.

I poked around and found this thread:

and this one:

Which basically says “It is Vertex Normals, not smoothing groups/face normals. Edit Poly will always discard them… Any topology changes or topology changing modifiers will discard them.”

I added Edit Normals modifier and that eliminated them. I tried the Collapse utility and that eliminated them as well (though, I’ve never used that before so it’s quite possible I did it wrong.

Those threads are from 2011 and 2012 so I’m wondering if this issue has been solved. Is there a way to preserve smoothing group on CAD geometry that has Vertex Normals?

Thanks in advance.


I’ve been forced to deal with this a ton the past month so I’ll share what I learned. You have to handle cad files with kid gloves in order to preserve those precious normals! To detach a piece, duplicate the entire model, and delete everything but that piece, and then in the orig model, delete that piece.

In order to attach one model to another, first run the reset xform utility on both models. If you change the orientation of any of those models later and need to do more attaching, run that utility again!

If you attach a model with smooth groups to your cad model, the faces with smooth groups might temporarily have incorrect shading. This can be fixed by flipping the faces and then flipping back again, or sometimes they fix themselves when you attach additional models.

Make sure none of your models have transform keyframes, or are patented to objects with said keyframes. If so, the reset utility won’t fix your problem.

As you found out, many modifiers, including edit poly and edit mesh, wipe out the normals. So if you need to add geometry that requires a complicated modifier stack, build it as a separate object, and then attach it using the principles I mentioned at the editable poly/mesh level.

If you welded new faces that have smoothing groups to existing cad geometry (for instance to patch holes), you can blend the seams by applying the edit normals modifier and copying and pasting the normals along the seam.