Hi Petr, almost forgot this second part:

> Blend :: When I make a blend (G1, G2) between two curves then join this three curves
> into one and extrude it (or use it as a path or profile for sweeping), the result is
> polysurface rather then smoothly deformable surface. Is it useful? (Yes, I can use
> AddPt and Delete commands for removing a kink from curve before extruding...)

I don't know about useful or not, but that's basically how things have to work to get exact results.

MoI doesn't try to fuse G1 or higher segments together into single curve pieces when doing an extrude, for example. This is different than what you might be used to from Rhino - Rhino will fuse G1 or higher segments together into single curves when extruding, but this actually tends to be a source of problems, for example: http://news2.mcneel.com/scripts/dnewsweb.exe?cmd=article&group=rhino&item=251873&utag= , it can also be inconvenient for other situations, like it makes it more difficult to explode or ExtractSrf your model into individual faces for low level work. So I consider it a feature that MoI does not auto-fuse G1 segments together.

Also, even though Rhino fuses G1 segments together, that does not equate to "smoothly deformable surface" - the only general way to fuse segments together is by introducing a "fully multiple knot" which becomes a corner point if you drag it. So "fusing" does not equal "smoothing".

In general making a smoothly deformable surface out of multiple segments will involve some type of refitting or resampling process to reconstruct the curve without internal corner points (fully multiple knots). This involves small deviations from the original shape, probably not very expected for extrude, but probably more feasible for sweep profiles, especially profiles other than the start/end ones.

So basically if you want a smoothly deforming single surface from an extrude, you will need to do the addpt/delete stuff to provide a single smoothly deforming curve first before extrusion.

For extrude, the structure of the input curves exactly dictates the structure of the resulting surface. I think this is the most expected result - each curve segment = one surface.

- Michael