Design of a Faded Surface Crease

I’d like to ask if anyone has advice on how to approach the following design challenge. I’ve tried various techniques without a clean result.

Given an existing curved surface, how would one go about building the curved crease shown below (on the dark red surface) into said existing surface? On either end of the crease, it fades into the surrounding surface, and the crease’s ‘dropoff’ is most pronounced toward its middle.

The surface I’m starting from is attached, with a green line representing the location of the desired crease. Many thanks for any advice on how to approach this.

HelmetCrease.3dm (2.3 MB)

Hello - is this (quick and dirty…) about the right idea…?

HelmetCrease_Maybe.3dm (1.1 MB)


This is close, although my goal is to have the blue surface share the same curvature as the green surface. However, the lower sweep you created (in red) is as intended.

Maybe I need to somehow offset or re-blend the existing green surface. In the original image I attached, you’ll notice that the desired surface in red does not have any “wobble” above the top edge of the crease. Thank you!

HelmetCrease_Maybe.3dm (1.3 MB)

Hello - a little more surface matching will get you there -

Looks like I only matched for tangency maybe. In the position I have it and at the ‘elevation’ I have it, there is a reversal in curvature of the top piece but if it were a little lower, or a little farther down the side, the green surface could extend and not reverse to hit the crease curve.
This is still pretty crude but it is the right approach I think:

Some variation on:

  1. make two curves that bound the area of the crease - can be isos in this case
  2. OffsetSrf the base surface out by crease depth. Pull the shape curve to the surface.
  3. BlendCrv to the base curve that describes the crease shape. One at each end to fade to taste.
  4. Trim and join these to define the 3d ridge.
  5. Loft the curves from step 1 and the new ridge curve, with Straight Sections and possibly Rebuild with enough points
  6. Explode the result.
    7.MatchSrf the outer edges of these for Curvature with the other end set to Position.

I’ll make you a file tomorrow…

@fluxguys - here’s how I think I’d do this:

Create the two parallel curves and the ridge curves and loft to make two strips.
MatchSrf with History the outer edges of the strips for Curvature and keep the other end Position, so you get a total of four points across. BUT, when you match, set CurveNearSurface to on and skip picking a target curve, so the edge just gets sucked onto the surface. Match by closest points and ‘Preserve isocurve direction’
Then carefully move the edge points out where needed to extend the surface along the target surface with History keeping it matched. This way you can make the surface wider, where needed, especially at the back, to avoid the revesal in curvature.

Using DragMode set to ControlPolyon you can easily move the outer points outward, and the next row out a bit etc, all the time maintaining curvature with the target surface.

I’ll make you a little clip if that is not clear…

HelmetCrease_Maybe.3dm (1.1 MB)

There is still some tuning to do on the shape but this will give you an idea how to go, I hope…

Also, once you like it, MergeSrf on the two strips with Roundness=.25 (or so) will nicely kill that hard edge. So no one gets hurt.


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Beautiful. I’ll give it a shot. Thanks as always Pascal.

Extremely helpful, and clear all the way through. My one and only point of uncertainly is regarding the blue surface, which is a result of one of the two lofts you made. Did you rebuild that surface after lofting/matching? When I do the initial loft, “refit within” and “do not simplify” both create too much complexity for me, and using “rebuild with” seems too arbitrary (Style is set to ‘normal’). Can’t thank you enough for the quick schooling.

The blue surface is one of the initial lofts, and yes, in this case I used Rebuild, with I think, 24 points - it looked like enough to pick up that ridge curve reasonably and not have too many points to do the by-hand adjustments along the outer edge. If when it’s all done there is not enough to match up to join, you can use InsertKnot > Automatic to add complexity and match more exactly, if you have no more point-noodling to do.
The main thing to notice here is that you can adjust the reverse curvature out but sliding that outer/upper edge of the blue surface farther out on the green one where needed.


Points well taken. Thank you.