Hi Mike,

> Just for development notation: I intuitively tried to hit the delete key
> and the "X" to make a knot disappear. (wishful thinking) ;-)

Which knot would disappear though? Since there isn't currently any way to select a surface knot line there isn't a good way for me to know which particular knot line to remove when you push delete, that's the problem...

It probably needs a special command for it that shows you the different knot lines on the surface (which are certain surface isoparm lines, in between surface spans) and let you pick those.

- Michael

Ok...

Michael, you'll have to forgive me for my lack of understanding, so the rows or columns represented by the points are not knots? Oh boy. :-/

Hi Mike, Control points and knots are different things, both of them are things that go into the definition of the curve or surface though.

Knots set up what are called "basis functions" which is one thing that controls how much a particular control point affects the curve like if it pulls the curve more strongly towards the point or not. Then just to be even more confusing there is also another thing called "weights" that go on control points that also has a kind of similar "strength of pull" effect as well with the main difference being that the weights can pull in just the exact way to make conic section shapes like a circle or ellipse.

But the main type of thing that is usually removed for a NURBS surface is a knot, which then has a side effect of also removing a row of control points as well.

- Michael

""""so the rows or columns represented by the points are not knots?""""""""

Here's a link to a good description thats understandable:

http://www.rhino3d.com/nurbs/

Look at the 3rd and 4th sections under "What is NURBS geometry"....

Ahh... the venerable and all-important "filler" inside the hot dog required to make them "plump when you cook 'em". ;-)

And not so easy to understand:


Knots and Control Points
A common misconception is that each knot is paired with a control point. This is true only for degree 1 NURBS (polylines). For higher degree NURBS, there are groups of 2 x degree knots that correspond to groups of (degree+1) control points. For example, suppose we have a degree 3 NURBS with 7 control points and knots 0,0,0,1,2,5,8,8,8. The first four control points are grouped with the first six knots. The second through fifth control points are grouped with the knots 0,0,1,2,5,8. The third through sixth control points are grouped with the knots 0,1,2,5,8,8. The last four control points are grouped with the last six knots.





Maybe what I need to ask for is a way to remove control points.

Hi Mike,

> Maybe what I need to ask for is a way to remove control points.

That could be possible, but there are some complications - the number of knots and control points are related, so if you remove a control point you also have to remove a knot as well and then you have to decide do you want the other control points to shift in position or do you want the other control points to stay where they were which instead could make the surface kind of mutate strangely in shape with a shifted knot vector.

If you want to do a lot of surface control point manipulation, sub-d modeling tends to be a better tool for that...

- Michael