Complaint by Steph

 From:  Michael Gibson
170.25 In reply to 170.21 
> Pss Silly Question : After a Chamfer or a Filet of an object "Show Points" don't work!
> Is that normal? (I am a little tired with this winter / summer hours changes:)
> I must separate again for show it!

It's normal - it is a limitation of how NURBS work. You can only turn on points for an object with joined surfaces, if all the surfaces have the same control point structure along their common edges, and the common edges must be "natural edges" of the surface, not trimmed edges.

When you have a trimmed surface (which is the result after a chamfer or fillet operation), the control points of the underlying surface are different than the structure of the edge. This is what is really different between NURBS modeling and polygon modeling - in polygon modeling the edges you see are always the edges of the surfaces that are involved (they are one and the same). This is not always the case with NURBS modeling, here is an example:

Here the surface is a simple plane made up of 4 control points. The plane has a trim curve on it that trims away a portion of it. But the points that make up the trim curve don't define the surface, the surface is controlled just by the 4 surface points.

That's why in general you can't manipulate NURBS surfaces by dragging their edges as you would in a polygon modeling program - trimmed edges define regions of the surface but do not define the shape of the surface itself, if that makes any sense.

But on the other hand, this method of defining a surface is the reason why booleans with NURBS is so tremendously better than with polygons - every time you do a boolean or a trim, the surface stays exactly the same and just new trim curves are calculated. It's much more of a "nondestructive" type of operation. You can even recover the original surface by removing the trims later on if you wish. With polygon modelers, when you do a boolean it generates a tremendous number of new facets all over the place, the complexity of your object starts to increase dramatically.

So now imagine that I have another surface attached to the trim curve of that plane - for example an extrusion of the trim curve. This second surface has a totally different set of control points than the plane does. If I allowed you to turn on the points for such a joined set of surfaces, it would be really easy for you to grab one of the points and pull it such that it created a gap between what used to be a shared edge. This is why such point editing is not allowed, it is too easy for it to create gaps between shared edges.

Please let me know if any of this doesn't make sense.

This understanding of how trimmed surfaces work is really a key idea in understanding why NURBS modeling does not follow the same type of face/edge/point dragging as poly modelers. It's a very different structure and they have very different ways of working. Each way has its own particular strengths and weaknesses.

- Michael