Stuck on Flow Along Surface (FlowAlongSrf)

I am trying to learn/use the command FlowAlongSrf, and am using the below tutorial;

Reference Tutorial

I am stuck on the “Base Surface - select edge near a corner” and cannot advance. In the tutorial they just click on the corner of the surface created using planar curves and then they are prompted to select the Target Surface. When I try to recreate it I cannot move on.

I have attempted to display this below. My screen is at the top, and the tutorial at the bottom.

I DID search the forums and found this post, which seems similar to my problem, but because the attachments are not available I do not understand what I am doing wrong.
Similar Forum Post

Any help is greatly appreciated.
Flow Along Surface.3dm (1.6 MB)

Hi Nial - see if this clip helps at all - (I’ve grouped the spirals.)

Note you get some distortion due aspect ratio on the revolved surface.

You can mitigate this by making the base surface a little taller and tapered to match the taper in the revolved one.


Pascal, thank you. I am very new to Rhino, so please help me understand my error. I grouped my spirals this time and tried again. It seems to work.

Was my error that I was drag selecting ALL the objects for the FlowAlongSrf, and then was not able to select the corner. What I really need to do is select only the objects I want to flow, and then have a separate arbitrary surface that is used only for the selection?

Am I over-thinking it?

Hi Nial - grouping is just a convenience - it should work either way. One thing to understand about FlowAlongSrf is that it maps from the UV and normal of one surface to the UV and normal of the other - so if these do not match up pretty well, you get distortion or unexpected orientation. Running the command with History on, and then modifying the inputs is a good way to get the results you want. If my objects are complex, I usually do all the adjustment with curves only, so they update quickly, then when it looks correct, I do the actual objects - seethe attached file - scale the base surface a little with gumball and see what happens to the flowed objects.

Flow Along Surface_PG.3dm (591.4 KB)


Nial, I think you are correct - you need to only select the objects to flow, not the surfaces that are under them.