The rules for SubD can be pretty tricky…
Imagine that this is the edge of a car fender. I want to control the highlighted internal-edge/seam, shown below.
But it I a seam, it snarls the fender lip. The way SubD does its thing, every surface’s edge pulls on the node, affecting the position of the virtual smoothed node.
By removing the internal-edge/seam it restores the fender edge:
This shows the effect of the additional split/internal-edge.
What if we could…
1.) Click on the end of an internal edge near one end.
(Doing so tells Rhino which end of the edge we’re talking about.)
2.) Run the command SubD_Untieend, or Slipknot, or Softend, or something like that.
3.) The internal edge you selected is relaxed, as not affecting other other seams as if the seam is not present, but the opposite of the internal-edge/seam is still constrained for the smoothing as if the edge is present.
The effect would be greatest near the Untied end, tapering to nothing at the other end. The smoothed/virtual node on the left would be changed. The node on the right would not move the one on the left would.
The result would look like this crude PhotoShop approximation. I do not think that this is possible, presently.
4.) The edge could be restored to normal by using a SubD_Tieend or something like that.
On the way to T-SubDs and Boolean-Composite SubDs… : )
Some of this can be worked around, but it makes the result deviate from what I had wanted.
