Blend Surface history inverts directions in Rhino 8

Hi. Has anyone encountered this problem?

I create a regular Blend Surface and history inverts the directions on it.
I’m not sure it supposed to work like this.
Tested on Version 8 SR7
(8.7.24082.18001, 2024-03-22)

Btw It is ok in Rhino 7 though.

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It looks like BlendSrf with History problems are well known: BUG: Issue with RH-53677 (BlendSrf)

Can you send the files you are testing with? I am tracking these.

@scottd It seems easy to reproduce, see attached. It does not seem to be caused by flipping directions but happens if something other than natural is chosen, so either manual adjustment or Automatic. Rhino 7 behaves normally.
history_blendsrf_messes_up.3dm (126.5 KB)
history_blendsrf_rhino7.3dm (876.8 KB)

edit: this seems to be fixed already in the latest 8.7

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Sure. Here you go

I have found that the problem appears on Periodic surfaces.

Blend_Surface_Problems_test.3dm (3.4 MB)

If the surface is created from an open curve, History works fine.

The only way to fix the problem with History in closed surfaces is to break the edge and create a Blend Surface in a few steps.

I am sending a file where I have shown how this problem looks like on various examples.

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hi @Dmitry_Medvedew thanks for sending the samples. These seem to work well now in the latest internal 8.7 build (but the blend surfaces have to be reconstructed to work properly)

When will the SRC with 8.7 be available to mere mortals - Tuesday April 9?

normally, yes

We did make some fixes to 8.7 based on the existing reports. Although I am still collecting examples as this history blend issue has been around for a while. Some of the issues were a problem in blend that is think we have fixed, namely the flip problem. But some other of these issues may be that the topology of the curves is changing due to the edit. We need to separate these two issues. Not sure what we can do with topological changes.

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OK, Houston, we have another problem.

I extruded a flat curve (it doesn’t matter if it’s open or not), created a BlendSurface and tried to rotate the curve. So it is broken again.


If I rotate the surface, then everything is fine. So I suppose this is also a history issue.

Blend_Surface_Problems_test3.3dm (602.8 KB)

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Yeah. Good example. I will add it to the collection. We will give that a test.

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So my internal build is working for these examples. That is good news. I believe tomorrow this update gets released atleast as a RC. So we can all start testing tomorrow.

We still can make it fail, but it also works in more cases. It is going to take more investigation.


hi Dmitry,

Mikko is looking to see if this one can be fixed, but this seems tricky, because moving the curve in the way you demonstrate will turn the extrusion object into a brep, which in turn changes the edge indices.

Quoting Mikko:

An easy fix is to avoid using extrusions. Set UseExtrusions to Polysurface and extrude and blend the surfaces again, and history will update properly

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If the Extrusion command options is set to create a BREP initially then this break in topology will not happen when rotating the curves that breaks the blend.