Hi Burr, also this description may help a bit.

It's basically a fundamental thing to NURBS that there is a kind of weight already associated with each control point.

The whole way that NURBS generates a shape is by actually blending together the control points in different amounts as you travel along the curve.

Right at the start of the curve, the shape there is defined 100% by just the end control point like this:



As you start to travel along the curve (which basically means moving in "parameter space" along the curve's parameter values), the proportions of influence of each control point changes. So for example at this point in the curve shown at the arrow here, the points are blended by something similar to the percentages shown:



The blending means to literally take that amount of each control point's location value - like the x value of the curve at that point is calculated by taking the start control point's x value multiplied by 0.5 and then adding to that the the second control point's x value multiplied by 0.4 and then adding in the third control point's x value multiplied by 0.1 and then the fourth control point's x value multipled by 0.05 .

So anyway the constraint mechanism will alter the control points by that same kind of weighting that they are influencing that point on the curve. So for example if you do the "drag point on curve" by dragging on that arrow spot in the second image, the first 2 control points are going to move around mostly with your drag and the third one quite a bit less and the fourth one barely at all.

The fifth control point is not used at all in the calculation of the curve's shape at that arrow spot, so it is also not moved around at all when doing a "drag point on curve" from that location.


I didn't check the exact percentages for that diagram, those are just something generally close to give you the idea. I think that in this actual case the first 2 percentages are actually closer to equal like 40% each for that specific case.


- Michael