Update documentation.

e41b4c2a · Ilya Portnov · a5dc101f · e41b4c2a · e41b4c2a
--- a/docs/data_structure/nurbs.rst
+++ b/docs/data_structure/nurbs.rst
@@ -96,6 +96,16 @@ for reference.
  in which distance between neighbour knots is different in different places.
  For example, ``0 0 0.5 0.6 1.5 1.7 1.9 2 2 2`` is a non-uniform (and clamped)
  knot vector.
+* **Node values**, **Node points**. Also known as Greville abscissaes and
+  Greville points, average knot values, or simply edit points. The values of
+  NURBS curve's T parameter, defined as: ``t[i] = sum(u[i+j] for j from 1 to p)
+  / p``, where ``u`` is curve's knotvector, and ``p`` is curve's degree.  NURBS
+  curve *node points* (or Greville points) are points at the curve at parameter
+  values equal to node values.  The number of curve's nodes is equal to the
+  number of curve's control points.  Node points of the curve are positioned on
+  the curve near corresponding control points. So in many NURBS algorithms,
+  node points are used to define the shape of NURBS curve, instead of control
+  points.
 .. _Book: https://www.springer.com/gp/book/9783642973857

--- a/docs/nodes/curve/nurbs_curve_nodes.rst
+++ b/docs/nodes/curve/nurbs_curve_nodes.rst
@@ -5,7 +5,7 @@ Functionality
 -------------
 This node calculates so-called *node* points (also known as Greville points or
-average knot points) of the NURBS curve.
+average knot points, or simply edit points) of the NURBS curve.
 NURBS curve *nodes* (or Greville abscissaes) are defined as:
 ``t[i] = sum(u[i+j] for j from 1 to p) / p``, where ``u`` is curve's knotvector, and
 ``p`` is curve's degree.
@@ -34,3 +34,7 @@ This node has the following outputs:
 Example of Usage
 ----------------
+Orange is curve's control polygon; dark blue are curve's node points (Greville points):
+.. image:: https://user-images.githubusercontent.com/284644/186223165-72a48126-b290-4eb9-a3cc-262fd23bf426.png