Thanks Martin - I think the blend surfaces may be missing in SAT format because they're probably specified as a "logical blend" where instead of storing the actual NURBS surface what is stored is just a kind of tag saying "make a blend between these edges" and then the receiving application needs to calculate the actual blend. You may be able to set some option at export time to stop that from happening and having it write out NURBS surface data for everything, possibly by specifying an older SAT file version (maybe ACIS v8 or v7 or something along those lines).

The main thing that I wanted to check was surface quality, and there are several problems with the blend surfaces generated by ViaCAD, with some ripples and bumps and lack of tangent continuity... It's difficult to see these problems in ViaCAD's display since it tends to use a pretty rough triangulation which tends to make it hard to really analyze the surface quality very well.

You can see some of these problems in these areas here (all following screenshots in MoI):




It's possible to get a really really clear image of every nuance of a surface in MoI by exporting to a polygon mesh format like OBJ format, and then cranking the mesh density up super high so that you can be sure that there are plenty of polygons being used in every nook and cranny of the surface, that will then show you the surface in super high definition. So for example I cranked the mesh export up to this level of density:



All those little tiny black lines are the outlines of little teeny-tiny polygons, this is how you can be sure that you're seeing a really good image of the surface without polygonal artifacts coming into play and obscuring things. With every polygon about the size of one pixel it basically eliminates any polygon shading blending artifacts. Then when turning off the edge display and and only showing the shaded result you can see this is the super clear view of that same surface:




There you can pretty easily see the problems near the boundary edges - there's a bunch of bumps and the patch surface does not meet up with the adjacent surfaces totally tangent, there's a sharp area where they meet, particular at the top and bottom juncture areas.

There are similar problems with every single one of your other examples, here are some screenshots:












I've also tried some various test cases in ViaCAD myself and seen these same kinds of problems too, so I thought I'd ask you for your file to see if your own examples were of good quality or had these problems.

This is kind of troubling in general because I've sometimes thought that maybe I should license the ACIS geometry kernel to improve filleting in MoI, but if this is the best quality that the n-sided patch mechanism in ACIS can deliver I'm not sure that it will really be that great, although I guess an argument can be made that it's better to have this than nothing... and also some kinds of usage purposes like if you're making an STL/3D printed result the size of the lumps are small enough that they're probably less than the size of the printer resolution itself and so probably don't matter for that case. It's not nice for high quality rendering though.

It kind of turns out that MoI's high quality surface display is almost something of a curse in this situation - just the regular display without doing the high density export generates enough triangles that you can see these surface defects during regular modeling, while other CAD programs have such a lower density triangulation that these defects get obscured and many people are just unaware that there's a problem until maybe they try to export something to a renderer and make it reflective and see that it looks bad in a close-up in the renderer...

I guess that possibly a good n-sided patch mechanism is rarer than what I had previously thought. Or maybe just all these cases are just too difficult on it to handle very well, although even just doing a setback corner on a box has problems. The file valise.3dm posted at the very first message of this whole thread has a whole bunch of lumps and bumps all over it...

- Michael