What's the best approach for creating this transitional surface?

I’m starting to play around with surfacing a really smooth “futuristic” wheelbase concept for fun. I’m specifically trying to maintain principals I’ve read on here, like:

  • Keeping everything single span
  • Sticking primarily to only deg3 and deg5 curves/surfaces
  • “Sculpt and match” versus getting too “curves in / surfaces out”

But I’m already running into walls and could use some help.

The first big question is what’s the best way to approach filling in this transitional surface area? I think it’s pretty obvious/intuitive what the missing surface should look like in theory, especially if you look at the file — so it’s more a technical question than a creative one. The question is, how do you build this without cheating and using a patch tool, etc?

I’ve tried a dozen ways of doing it and just can’t make it work. I’ve tried extracting isocurves from neighboring surfaces and then creating a set of blendCrvs, and then using EdgeSrf to fill in planes, but the curves and surfaces never quite transition properly, and it’s seemingly impossible for me to match them.

I also can’t figure out the best way to avoid 3 or 5 sided surfaces in this area. I’ve watched plenty of tutorials on how to creatively find 4-sided surfaces, but struggling with applying that general idea here.

The second question is why are these 2 surfaces showing glitchy breaks like this? On paper they should be seamless:

  • The surfaces have the exact same control points
  • I’ve run MatchSrf countless times on them
  • The edges of the surfaces are G2 continuous and I’ve checked a dozen times…

So what’s the problem… why are they not rendering as watertight?

Here is my file for anyone interested. Thank you!

help.3dm (953.0 KB)

I’d try something like this to start-

I was going to suggest this sort of thing:


I like that better-

I was thinking something along the same lines. It’s a blend between two trimmed egdes, but if you spend some time setting it up right, you can get a pretty good result.

help_Mark.3dm (1.2 MB)


The usual cause - coarseness of the render mesh. Refine the mesh sufficiently and the apparent gaps vanish.

Three different solutions for the blend surface(s) and three different shapes. What is the design intent? Is there a design intent for the blend beyond it being smooth and “fair”?

help.3dm (648.9 KB)

different solutions. There is a lot more. (btw. check: _Dir)


Definitely a situation where a trim is preferred over a 5 sided explicit patch solution. Lots of ways to skin this cat, mine is attached. Two patches keeps the trimmed edge smaller - I also revised your input surfaces, there was more of them than necessary imho.

help_SG.3dm (425.5 KB)


As is always the case with Rhino surfaces, Join the surfaces and the gaps actually vanish in the render mesh, which is not a great way to look at any kind of adjacency anyway.

Adjusting mesh settings can reduce this effect.

Default mesh settings joined surfaces:


I got bored so I polished it up even more. G2 now, nice smooth curvature combs. God I love VSR.

help_SG_G2.3dm (1.6 MB)

@Rhino_Bulgaria This model is better - still not what I’d consider “Class-A” but at least G2 at all edges, according to VSR. It’s entirely different surfaces from the previous one.


Sorry for uploading a gazzilion screenshots, but my Rhino 6 evaluation expired and I can’t save the model. I used @sgreenawalt’s model as a base for my little tutorial. He did a very nice job. I would do it in a similar fashion, except that in my example the cutting shape closely resembles the straight extruded body to make a more natural blend with the round body.


Hmmm, I noticed that the Zebra analysis of Rhino 6 revealed some deviation at the upper area. Can you confirm if VSR considers these edges to be watertight and at least G1?

Also, there was a G2 continuity between the upper and lower surfaces, whereas it had to be something like G1,5. The reason for this is because G2 usually affects the 3rd row of control points too much in situations where the matched surface is expected to connect to a G1 surface (both, the straight extrusion and the round body are G1). I explained this relation in another topic recently where we had a discussion about the Class-A modeling. I fixed that with my favourite Rhino tool “Move UVN”.


I”ll have to try your method @Rhino_Bulgaria. Had not seen that approach, it’s really clean. I also really like how @sgreenawalt split up the transition. Very cool to see the different approaches :+1:

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Are you looking at the first file I uploaded or the second? The first pass was pretty crude, just to show patch layout/feasibility. The second is better - according to VSR it’s G2 all around.

I looked at your 2nd model. I just tried it again with the Zebra. It shows some visible deviation. Maybe the set tolerance needs to be tighter in VSR?

As for the G2 case, as I mentioned above, if the adjacent surfaces are G1, the surface to match must follow the same principle. G2 will make the transition overly-smooth, which destroys the G1 flow of the original surfaces (the straight extrusion and the round body).

Honestly this is why I think discussions about continuity that are based in zebra analysis are kinda pointless. :man_shrugging:

The Zebra analysis reveals visual deviation. Curvature continuity also confirmed that. Not sure why VSR considers this to be watertight G2.

As for the continuity between surfaces discussion, having G2 does not guarantee smooth flow or smooth reflections of the model. G2 only gives a mathematically correct smooth transition of the 2nd and 3rd rows of control points, whereas the optical quality depends on the overall flow of the connected surfaces.

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This is a work of art @Rhino_Bulgaria. Bravo!


You can see from my post above the values that VSR spits out. Look at edge #3. The edge in question is G0 to 0.0001 and G1 to 0.17 degrees and G2 @ 0.07. I never said this was a class A model, I said it was G2. You’re sorta splitting hairs here - in fact essentially what you’re saying is if I point edit those verts it’ll be G3 across that centerline. Okay, sure, but I never said this was G3 and I never said this was Class-A.