Triangle surface in Rhinoceros

Hey guys,

I really like Rhinoceros software and I wouldn’t change it for any other, but I can’t get over issue with triangle surfaces. Maybe it’s silly, maybe I’m thinking about it in wrong way, maybe answer is just one sentence, but for sure: I’m trying, but can’t get over it for a week.

Let say, I want two surfaces (of car side, or something); top one is quad, the second one is triangle. Just like here:

I love to use _EdgeSrf and of course I will use it here; the quad is made in notime (and it’s looking perfect), but there’s problem with triangle one.

It’s reflections looks bad. Just as mesh:

I could try to create it as quad like here and cut it later (mesh would be right), but the edge of top surface won’t cover with diagonal of bottom surface:

It really drives me crazy. Any idea how to make it right? I would really, really appreciate detailed explanation…

Hi Otto- the trimmed version is the way to get a surface that has good curvature- the original triangle version has a compressed UV structure and will have the curvature follow along, so to speak. Here is a way to get what you are after, I think:
1.Make the 4 sided surface that you would like to trim into a triangle.
2. Start Patch and select the three sides of the triangle.
3. In patch, set the Starting Surface as the four sided surface, Pull=0. AutomaticTrim = checked
4. Depending on the structure of the starting surface, you may or may not get to set the spans in Patch. If the shape is simple like this set it to 1 and see how that looks.

Any luck with that?


Another way is to split one of the curves of your triangle (I’d try the longest curve, but YMMV) in two pieces of more or less equal length. Then, use the 4 curves as in put for EdgeSrf rather than the 3 curves. This will lead to a differently parameterized surface and differently shaped mesh, that may not exhibit the reflection problems you see with the 3 curve based surface.

Hi Menno- you can get a surface this way but having adjacent edges tangent on a surface should be avoided- the point where U and V become parallel in such a surface is hard to deal with- surface normals, which depend upon cross products of the U and V, become undefined there. Trimming a rectangle is a much better bet, structure-wise.


I made it as you sugested (I think), but still it’s not perfect. Anyway, your post made me think about other way to get what I want. First, I’ve made simple quad surface, trimmed it at diagonal, and then - using the PointsOn command and Snap to point - get the points right. Later I’ve manipulated around longest edge of triangle to get curve right. Here it is:

Do you think it’s good idea, or there are cons (or “contraindications”, I’m not sure what I should use in English) that I’m not aware of?

Hi Otto- try changing the degree of the quad surface to 3 by 3 or 5 by 5 and then use it as the starting surface- any better?


@pascal, this is pretty common problem, and can get quite complex. Is there anyone at McNeel that might could to some tutorials for approaching 3/5 sided surfaces with trims?

Hi Eric- here is an example file reproducing, approximately, the situation posted.

TriangleSrf.3dm (84.5 KB)


Judging from your images i assume that you try to make a wheelarch? if so, you are doing it completely wrong, im not saying this is the best way but its a way to achieve this, and if this is not what youre after, well then consider this post not sent,…

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Hey @pascal, tutorials like @Sabino just posted. That was fantastic. I can do easy ones in Rhino, but the more difficult scenarios I just need help with the approach.

Sorry to intrude but would like to know what the above is referring to.


Hi Ken- if you set up some curves like the ones in my expample file a couple of posts up and you make the EdgeSrf from the triangle of curves and also make one from the 4-sided set of curves and Trim the triangle with the diagonal curve- just open my file and work with that. When you’ve got these two triangular surfaces made, turn on points (F10) for both- see how the 3-sided EdgeSrf has its point grid compressed to one corner and the other, trimmed one has an evenly spaced rectangular grid of points? The latter is generally far preferable when it comes to controlling the curvature of the surface. Curvauture tend to increase rapidly as it approaches the compressed point - that is, one entire edge of the four-sided surface is compressed to a point, a ‘singularity’ in NURBS Speak.


Ok guys,

First of all, thank everyone for replies, I hope it will be useful for more than just me.

Pascal - I’ve set deegre to 3 by 3 (and 5 by 5 later) and it just won’t work in my case; I recive wavy surface. Maybe it’s just fault of fast drawed curves I’ve made.

Sabino - I’m not doing a wheelarch, anyway every tutorial is worth seeing :slight_smile:

I think that issue is solved in Pascal’s file. Thank you all!

By the way…
I somehow understand why Rhino does not draw three-sided NURBS pieces, but… however nice the 4-sided geometry is, still sometimes the restriction to quads-only polysurfs makes a lot of trouble with an automatic representation of shapes in a way of “random” G3-arranged topology.