T-Spline Tip: Maximizing Surface Smoothness around Star Points

If you look at page 50 of the current T-Splines manual, it has a really nice breakdown of the effect of different types of surface topology on the smoothness of the T-Spline surface.  As you can see, nearly all points of  a T-Spline surface are C2 continuous:

This means that they are curvature continuous surfaces.  The only exception to this is in the area around star points, which are G1 continuous, which is a lower level of smoothness, more like tangent continuous.  For the wheel pants I’m working on, there are four star points near the front, as this is essentially a box topology.

If I run zebra on the T-Spline surface, you can see there is a bit of a burble in the surface (Sidenote – the Zebra mesh settings run off of your T-Spline mesh settings, NOT the settings in your Rhino mesh or the mesh box in the Zebra command, so if you want your Zebra to look nice, you better go set your T-Spline mesh to high):

Now, this isn’t terrible, but what if I want it to be better?  I’ve been thinking about this a lot lately and have come up with a solution.  For reasons which will become clear, you should save this for the very LAST step before you convert your T-Spline to a NURBS surface.  So, let’s pretend this wheel pant is done, and what you see above is my final topology.  To minimize the disturbance to the surface, let’s first minimize the size of the faces surrounding the star point.  How do we do that without disturbing our surface shape?  Very easily – tsSubdivideFaceExact is your friend here.  If I subdivide my entire surface in exact mode say 3 or 4 times, I get this:

See why we have saved this until the end?  This surface is really not easily editable, since it’s so dense.  Oh yeah, I should mention, you should save a copy of your master surface first, so if you want to go back and edit, it’s easy.  As we’ve changed our topology you should also run tsMakeUniform, just out of habit.  Okay, so we’ve added lots of faces, but the shape is EXACTLY what it was before.  So let’s look at our Zebra analysis now:

Now, there is still a small burble there, but it’s FAR smaller than what we started with.  Also, I’ve played around with point editing the star point, and found that you can often tweak it ever so slightly to get it even smoother.  I suggest putting your manipulator into UVN mode and bumping it either in or out in the normal direction just a tiny bit to see if you can get it to disappear entirely.   Cool, no?

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