Smoother surface transition

Hello there! I need the advice of someone with more experience.

Basically, I want to generate a bulge within the light blue surface without changing the borders of the surface (marked in yellow). So far, the steps I’ve followed are:

  1. Drew two curves
  2. Loft the curves (straight section, no rebuilding surface)
  3. Intersect the middle to create a new curve and drew the bulge I wanted to create.
  4. Then I trimmed the curve to create 6 separate surfaces in total
  5. Then used Loft (straight section, no rebuilding surface) and use “ChangeDegree” to 9
  6. Finally used “MergeSurface” to make the buldge smoother and more used “BlendSurface” on one side, the other side I think can end as is without blending.

I want to know if there is a better way to do this? Especially because I would like the transition of the bulge to be a bit smother imagining that the original surface was extended, like in the second image shown in green, so it blends better into the sides, considering I need the sides the borders to remain the same. BTW I’ve already tried match surface match (both curvature and tangency) but it deforms the borders

surface merge test.3dm (508.7 KB)

Hello - I think it is easier than that - how about a new lofted surface through the ‘bulgey’ middle as a single curve -


Hello Pascal, thanks for the prompt response

Just to clear my doubts did you trim and join the middle curve or redrew it? Also what settings did you use for the loft?

Actually, one of the things I wanted to retain was the sharpness of the angles I just wanted to make the transitions to the edges a bit smoother. As reference in these images let us say if the “red” lines representer a hypothetical section of the straight loft I wanted to make it more like the “blue” line.

why not use subD?

Hi, benedict

I am not familiar with that command, how does it work? which is the full name there seems to be a lot of options when I type subD

Hi Richard - still shooting for the ‘easy way’ here - a Loose loft with curves like so may work OK.

SubD is not a bad idea…


Hey Richard,

usually Pascal is way more competent than me, but I’ll give it a try:

SubD is a new form of volume description that allows to combine easily sharpen edges and smooth surfaces.
there’s not a SubD command, but you can try subDLoft the same way you would do with loft (surface).
then select the edge you want to be sharp with ctrl+shift and then “_crease” which will change the edge from smooth to sharp (expressed very simple)

you need rhino 7 though


Damm that is exactly what I was aiming for, only I have a couple of doubts about how you made it?

I assume for the outer curves marked (circled in red) you just intersected the original surface, however for the centre curves (circled in blue) how did you build them? Did you redraw the centre curve or did you just trim and join the two curves I drew?

Also, would the edges highlighted in yellow be straight, I ask because I would like to join them to other loft surfaces that have straight edges?

Finally, and this may be my lack of knowledge in rhino, I thought that when you used rebuild and loose in “Loft” the edges where modified, that is to say the borders wouldn’t match with the curves I drew or would they?

Hi Richard - there is a bit of trickery here. The middlle curve is cobbled together from the curves you had there. I copied that curve over, on either side of the center but close together.
The outer curves are made from TweenCurves between the curves at the outer edges, and asking for something like 48 curves - I threw most of them away but kept four at each end - this way, I know I have a set of curves that are progressing, so to speak, towards the opposite edge, not just copies of the edge curves.

Then Loft > Loose, with Rebuild to a good number of points. I did not check the ends for straightness but my guess is they are pretty straight and MatchSrf can true them up to neighboring edges if they are pretty close.


Thanks Benedict,

I have Rhino 7 but I am still getting used to it. Actually I went from 5 to 7 without looking at 6. I haven’t heard of this command before, guess I’ll have to dig deeper into it. If you could post the settings you are recommending that would be very useful!

Thanks for this tip! It is much appreciated and if you know of someone who has posted a tutorial for this command that would be fantastic

SubD structure didn’t exist in RH6, it’s pretty new to me as well. I think there are tons of videos on youtube, I think one of the best is from McNeel Trainer Kyle Houchens:

it’s maybe too fast for subD beginners, but I’m sure the official rhino channel has some nice stuff

Hi Pascal - That was some great advice and so silly of me not to think about using “TweenCurves”, btw when you say you cobbled together the curves I had, what do you mean exactly? Did you use “BlendCrv” or some other technique?

Benedict - Thank you so much for the video! I’ll learn SubD

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Hi Pascal – just wondering after trying your technique I’ve been bumping against some results that I want to know how to alter. In the first case I think it is noticeable that the loose loft with rebuild does not have the same flow when compared to the straight loft, I was wondering if there was a way to solve this?

In the second image, and I think this is harder to see, the loft generates like a pull effect as if it is stating the bulge in a back position and thus generates a weird shape. I tried to solve this by altering the middle curve, just bringing it back but I want to know if there is a way to solve this without altering the curves?

I attached both problems in the rhino file, thanks!

surface loft loose test.3dm (378.0 KB)

Well, right… if the geometry varies like this from one end to the other then rebuilding will not always be helpful - for Loft, it will be best to draw your initial curves more carefully and all with the same structure - that is, from copies of an original. These curves seem rather more complex than they need to be, but I do not really know what the surrounding surfaces are like or what constraints you are working with.


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thanks, okey then I will try to just work over copies of the original. regarding your previous comment, when you said you cobbeled together the two curves I had, what do you mean bt this? Which commands did you use?

Hi Richard - in your original example, it was easy to cheat because the shape was quite symmetrical, so the rebuilt curves corresponded pretty well in how the points were distributed on the shapes - the newer one changes quite a bit, so points that are on a sharp bend on one curve, end up on matching to a point that is on a straight bit at the other end, so you get that flow problem. What you’ll need is curves that are organized the same way - three or four points on the initial straight bit, then three or four rounding the corner and then three or four more for the next straighter bit etc, whatever it takes, so that the same points on each curve describe corresponding features, if you see what I mean, then Loft can make each ‘feature’ flow nicely to the one on the next curve…


Thanks pascal! yes I get the idea of the workflow, what I also did was to break the curves into two parts at the point that was causing trouble and that fixed it, but I guess your method sounds more clean, I’ll try that as well!