Here are my tests:
1. Cylinder - original object -> seam visible
![](https://forum.corona-renderer.com/index.php?action=dlattach;topic=42815.0;attach=199084;image)
2. Cylinder - reconstructed in CAD -> no seam visible
![](https://forum.corona-renderer.com/index.php?action=dlattach;topic=42815.0;attach=199086;image)
3. Lens - original object -> heavy artifacts visible
![](https://forum.corona-renderer.com/index.php?action=dlattach;topic=42815.0;attach=199088;image)
4. Lens - reconstructed in CAD, imported as BodyObject, Face Angle 10 -> less artifacts visible
![](https://forum.corona-renderer.com/index.php?action=dlattach;topic=42815.0;attach=199090;image)
5. Lens - reconstructed in CAD, imported as BodyObject, Face Angle 1 -> no artifacts visible
![](https://forum.corona-renderer.com/index.php?action=dlattach;topic=42815.0;attach=199092;image)
The cylindric objects - mystery remains but it looks like a clean start resolves the issue.
As for the lens - yes, artifacts will be visible, probably due to the fact that Corona doesn't interpolate explicit normals (like it does for normal smoothing).
The only solution (or rather a workaround, not a solution) I found is to use high tessellation. If you import the original CAD file in an format that Max can read, you can dynamically change the tessellation to whatever is needed.
For this to work: Import file > Convert to Mesh = OFF, this will import the object as a BodyObject where tessellation can be controlled at any time.
Attaching a zip file with the max 2021 scene and the exported IGS CAD objects from MoI3d.
FWIW - @OP, you'll find similar problems with shading for non-transparent objects. Two options to handle this:
- again, high tessellation
- or: Render dialog > 'Development/Experimental Stuff' rollout (might have to be enabled in Corona System setting, enabled by checking 'enable devel/debug mode') > 'Terminator Handling' > set 'Shadow Shift' to 1 (0 by default). This will mostly fix shadowing artifacts on CAD models. Might however introduce shading artifacts on low resolution geometry using standard Max smoothing.
Guess development needs to have a look at properly handling CAD normals sooner or later. We have to use workarounds for quite some time now so please have some mercy devs :D