Why are blend surfaces sometimes flipped?

Why would this happen?

Is there anything you can do about it without killing the history?

I’m wondering if selecting both the base surfaces and doing a Dir > UReverse or VReverse (depending on whether the U or V direction edges were used for the blend) might result in flipping the blend back without killing the history…

Thanks for the tip, I did not know about that command before (but dear god, do I wish that Rhino would write the UV direction names instead of just color coding them, especially since although I’m not color blind, the red/green color blindness is apparently the most common).

Unfortunately, it didn’t seem to do much to improve the result of the command (recreated it as well, just in case):

Can you post your example (just the 2 surfaces, plus the history-enabled blend) for people to experiment with?


blend.3dm (73.8 KB)

Oh. My. God.

If I select the right surface first, I get a blend surface oriented in one direction, and if I select the left surface furst, I get a blend surface oriented in the other direction… :man_facepalming:

Interesting - the Blend surface does not seem to be history-related to the original surface UV directions or their normals…

Surfaces constructed with their edges running in the “counter-clockwise” direction generally have their normals facing upward. In playing with it, it also appears where you click on the edges that changes the direction loop.

Clicking on the upper-left then upper-right results in a up-facing normal, as does lower-right then lower-left. The other two choices result in a downward facing normal.

It seems fairly arbitrary actually, like the “auto guess” function in FlowAlongSrf - no way to know where to click in advance that I can find.