# Need help in understand:untrimmed surface edges

I’m watching the tutorial “Creating and Controlling a Transitional Surface in Rhino3d”: https://www.youtube.com/watch?v=3Eb7n-UrIoo
At 23:40 Phil Cook talks about “untrimmed edges” on surfaces.
I dont quite get it. What is a untrimmed edge and how do I convert a trimmed into a untrimmed edge?

Hi @JayR ,

Take a look at this link, where Trimmed surfaces are explained:
http://docs.mcneel.com/rhino/5/help/en-us/seealso/sak_splittrim.htm

http://docs.mcneel.com/rhino/5/help/en-us/index.htm#commands/untrim.htm

Hello @Rodri,
thanks for your comment, but I know what trimmed/ untrimmed faces are and the untrimmed command I know as well. I have worked my way through the training manual V1 + V2.

It’s about the big surface in the picture above. How can the red signed edge be untrimmed? Or is there a way to construct these smooth surface without trimming with a half rounded something?

I’m not sure if I follow you… untrimmed edges can’t be converted to trimmed edges, as far as i know. You can untrim the surface keeping the trim object, but I’m not sure that would help you much…

The “big surface” in your image cannot be constructed as an untrimmed NURBS surface. Untrimmed NURBS surfaces, due to the rectangular UV structure, must have exactly 4 edges - up to two of them can be collapsed into a single point such as the poles of a sphere - but they cannot have more than 4 “natural” edges. Thus that 6-edged surface must be a trimmed surface if it is to remain a single entity, otherwise a similar shape could be constructed with several 4 sided untrimmed NURBS patches (at least 3).

–Mitch

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I quote from the transcript:
“Another way of looking at this and particularly if you wanted a single span surface as a result of creating the transition surface is to make sure that both of the edges that we’re working against here are untrimmed edges and that this cross section and this cross section have the same number of control points.”

Don’t know how that applies in this case… --Mitch

Hej JayR,

typically, when building such kind of transitional surfaces in a controlled way, one does not build them to trimmed edges as shown in the video, but rather to curves of degree 5 or 7 (1st step), to then match them (step 2) to the surface they should transition from. So, I suppose, he means just that - building the surface to curves that deliver the shape seen from the side view of the car (for example, re-building the red edges as simple curves of degree 5 or 7) and then match the transitional surface(s) built from those curves to the adjacent surface(s). That’s how the car and aircraft modellers do it, to be able to always work with simple and easy to control four-sided Bezier (single patch) surfaces.

Hello @Lagom,

thank you for your explanation, even though I don’t really understand it yet. I think I still lack a lot of basic understanding for clean surface modelling. So I guess I’ll have to look for more tutorials about smooth transitions.

JR, Is Phil using the “Blend Surface” command in this case? —Mark
I guess I was wrong. He also used “2 sweep Rails”.

For this transition he draw 3 blended curves and used them with the 2 red marked edges for a 2rail sweep

Interesting… - if you watch right to the end of Phil’s clip, you can see how he manages to make the larger surface edge an untrimmed one - the surface is a radial one, with the ‘square’ edges trimmed off but the round wheel-arch edge the natural surface edge - it’s not the same surface as the one shown earlier in the clip…I’d say more often in that shape configuration, the wheel-arch would be trimmed - you could still manage the same operation by rebuilding duped edges and lofting and then matching. but it’s a bit more work perhaps.

-Pascal