Blend Surface - Symmetry

Hi all

Wondering if anyone would find a symmetry option when using blendsrf helpful?

So, if the shape has symmetry intent, then you can adjust the handles on one side and the other side updates in sync.

If doing blends (like the attached image, I need to manually adjust each quadrant, so in effect the result may not be truly symmetrical…Actually in this case, shaping one quadrant with the other 3/4 updating in sync. would be ideal. I can of course build each quadrant and mirror the result, but as you cannot control the tangency of the blend ends, there will be a kink between each quadrant!

Hi Sach - that seems handy, but maybe pretty hard to implement robustly - I don’t know if even reliably detecting symmetry to begin with is something that is very straightforward with Rhino’s rather free approach to surfacing.

@rajaa - what do you think?


I see where you’re coming from. Although, I don’t see another way to do this without creating the quarters then mirroring.

Sach, I think creating quarters and mirroring is a much more efficient strategy, especially because then you can add any trimmed details to one quadrant and when you want to propagate to any of the others you know it will match and be a closed solid. Also you really get to control teh surfacing the way you want it too look and your model will be a lot cleaner/lighter.

You can achieve good continuity with _Sweep2. If you do a beautifully light quadrant using 2 degree-5 curves with 6 points each (single span curves) you will get a nice set of rails.

Then using construction geometry or perpendicular curves you can build your rail section with _BlendCrv choosing your desired continuity: G1 or G2:

Then your sweep surface will be continuous across all quadrants

The middle will be a bit wonky at the first pass…

but surfaces a re very light to do very controlled point editing:

File here:
quadrant_2_rail_gf_220311.3dm (592.7 KB)

I hope this helps,


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Thanks for your input @gustojunk
Yes, I use a single-span workflow most of the time, especially for more refined modelling. But sometimes, I like to play with blend surface for quick, rough mock-up surfaces; I find it’s great for this, even though it produces a multi-span surface some of the time! I just want some more control within it!

Thanks @pascal
As I’m using this as a ‘sketch’ workflow, I can do it manually and get it visually close enough. But it would be very handy to have a symmetry option!

Hi @pascal - do you have any further news about symmetry when making Blend Surfaces? I note your comments about free approach to modelling. I find that in industrial design-type projects, there often are clear lines of symmetry. I think this function could be useful to many users?

From a user perspective, I’m thinking that if the input surfaces do indeed have symmetry, you could check boxes within the command window to lock it, for example, in the world axes. Then, when you adjust one handle, the shape change propagates to the other side.

Hi @rajaa - did you have any thoughts about this?