Why did MatchSrf change and already G2 surfce this much?

[eobet]

Since I’m used to a different software package, and trying to learn Rhino, I need my tools to be consistent and predictable. Right now, I’m having a difficulty understanding the following:

I’m preserving the direction, yet the surface after matching, deviates greatly from its previous location (red line compared to black).

On top of this, it was already G2 before I ran the command (this was just a test).

Can someone explain to me what is happening so I might understand what the tool is doing?

matchsrf.3dm (66.0 KB)

[pascal (Pascal Golay)]

Hello - only that it does the best it can I guess - @lowell will have a better idea what it is actually doing.
(no doubt you’ve figured this out, but It looks like a match for Position back to the curve does not mess anything up. )

-Pascal

[Rhino_Bulgaria (Bobi)]

Heavily distorting the matched surface sideways is undesirable and is something that was also discussed here:

Wish: "Match surface" to preserve the flow of control points

I wondered if the “Match surface” command could be upgraded to apply G1 or G2 to the general shape of a surface, while preserving the flow of its control points. The current implementation with curvature match (G2) forces the 3rd row of control points to follow the direction of the 2nd row. However, in many situations this will destroy the ideal shape of a surface. For example, Alias could apply curvature match that only adjusts the control points of the matched surface only in the normal direct…

A more elegant solution would be to have a matching algorithm that tries to preserve the flow of the surface and only moves the control points along their normal direction. Maybe an optional setting like this could be added in the “Match surface” panel?