Y-Fillet Surfacing / Topology HELP!

Hello All,

I’ve been stuck on this problem for a really long time and it’s absolutely driving me nuts!

Basically, I want to run two fillets/blends together and fade it out. (Please see attached “Original Geometry”)

The two fillets begins on either size of the horizontal flat triangle and eventually come together on a common theoretical edge.

As you can see from the diagram, everything looks good except the 5-sided missing patch in the center!

Hopefully, I want to attach G2 between all surfaces.

Any assistance or insight into this problem is much appreciated!

BTW, I’ve been working in Rhino3D.

Many Thanks!


The problem is the sharp connection between the two filleted surfaces at the end of the flat, triangle-shaped surface. Try trimming the ends of all three at once with a single curve so that their shared edges create a curvature continuous edge, then fill with a G2 surface.


Thank you for your suggestions! Here is my attempt at the problem.

It’s getting there but I still can’t get the resulting fill surfaces to be G2 on all edges.

Surface 1 & 2, had to be multi-span to match surrounding surfaces. I think it was Degree 5 with 12 CVs.

From the top view, notice the angle of line AB relative to the two fillets, i.e. runs perpendicular across the top fillet/blend.

Does this angle matter? Any rules in establishing the angle of this line?

I’m using Rhino3D and trying out VSR Shape Modelling plugin.

Kind Regards,


Could be of assistance: Y-Fillets in Class A Surfaces - Car Body Design

@Derek: not sure if you ever resolved this but if not, what I think engelhjs is trying to point out is his illustration is that it’s the singularity that’s causing the issue. If the surface is trimmed back so that there is no single point then it should be fine from there.