Hi tuananhnp,

re:
> It maybe hard to see the difference by looking at the screenshot. But upon close
> inspection the roughness SubD surfaces are quite apparent.

I think you're probably just seeing a display artifact in the real time viewport display where the display mesher is not making a lot of triangles in a few areas of your heavier model.

That's a normal type of display artifact and doesn't mean your actual model is rough. If you export out to a mesh format you can adjust the export mesh parameters and the "roughness" will be eliminated if you generate enough polygons.




re:
> Is it a limitation of the built-in SubD conversion? Like it could only smooth simple polygon?

No, it is just a mistake in your inspection method where you're closing examining the realtime viewport display mesh which is oriented around speed.

However, it is generally better with sub-d modeling to have as sparse of a control polygon cage as possible for several reasons - it's easier to control fewer cage points and data size will be less. When you have a high density control cage it becomes easier to have things like little bumps in the subd limit surface.

- Michael