I have a technical question about the edge we create when we trim surfaces ( green on my picture ).
It creates a too dense transition surface with too much control points, do you you know know if there is something to simplify an edge resulting of a “trim surface” and simplify this surface ?
thank you !!!

The complexity of the trim edge is a combination of the shape of the original object, the shape of the object used for trimming, and the absolute tolerance.

In general the intersection of two NURBS surfaces or the projection of a NURBS curve onto a NURBS surface cannot be exactly represented by a NURBS curve. (There are special case exceptions to this rule.) The result is Rhino when trimming, etc creates a curve with a sufficient number of control points to be within the absolute tolerance of the exact solution. Smaller absolute tolerance means more control points are required.

One method, in theory at least, to simplify the trim curve would be to change it’s shape so that fewer control points are required. This is complicated by the trim curve needing to conform to the surface being trimmed within absolute tolerance. I do not know of a method in Rhino to automatically do this. A procedure you could try (no guarantees): DupEdge the trim curve Rebuild or RebuildCrvNonUniform or FitCrv the duplicated curve to simplify it. I would probably use RebuildCrvNonUniform.
Pull the simplified curve onto the untrimmed surface.
Inspect the pulled curve. If it is simplified enough then Trim the surface using the new curve. If it is not simple enough rebuild the pulled curve, then Pull and Trim again.

Based on your illustration it appears that your absolute tolerance setting may be very small. Making the absolute tolerance larger should reduce the complexity of the trim curve if the model will be satisfactory with the larger tolerance. Remember that it is easy to make the absolute tolerance larger, but can require a lot of rework if it needs to be made smaller in the future. Also be aware that the absolute tolerance has zero effect on the accuracy of the math used in Rhino. It only affects the complexity of objects where the exact object desired cannot be exactly represented by a NURBS curve or surface.

Hi Enzo - I avoid using trim curves where possible as direct input. Here for example, I might make a blend curve, where your highlighted edge is but it will not be on the target surface - Loft to that and then MatchSrf to the surface with OnSurface=Yes. With History, you can make a nice adjustable blend, and make it more complex as needed to get right onto the target surface. Trim only once it looks good.