NURBS is abbreviation for Non-Uniform Rational B-Spline and it is a mathematical model to describe geometries in space. NURBS is one of the most popular mathematical models of geometries and used in many CAD/CAM software.
A NURBS geometry is defined by the following.

• control points
• weights
• knot vectors
• degree

Control points are an array of 3 dimensional coordinates for a NURBS curve, or a matrix of 3 dimensional coordinates for a NURBS surface. A weight is a positive number which can be assigned for each control point and it is usually 1.0. Knot vectors are an array of number used in the construction of the basis function shown below. A degree is a positive integer.
A NURBS curves is defined by a formula below, where P_i is i-th control point of total n control points, w_i is i-th weight, N_i,d is i-th NURBS basis function of degree d, which is constructed with a knot vector and Bernstein polynomials. The formula below is a function of a parameter u and it returns spatial coordinates corresponding to the input u.

Similarly, a NURBS surface is defined by the formula below, where now control points P_i,j and weights w_i,j are provided as n x m matrix. This is a function of two parameters u and v and it returns spatial coordinates corresponding to the input u and v.

For more detail description of NURBS geometries and basis functions, see Wikipedia pages of NURBS, B-Spline and Bernstein Polynomial

As we've already seen before, NURBS curves and surfaces can be created with ICurve and ISurface by providing an array or a 2 dimensional array of IVec and degrees. When you provide IVec, weights of NURBS geometry is all 1.0. If you want to specify weights, please use arrays of IVec4 which has w field as weight in addition to x, y and z field. Knot vectors are created automatically inside ICurve and ISurface and they are always unitized between 0.0 and 1.0.

add_library('igeo')

size( 480, 360, IG.GL )

controlPoints1 = []
controlPoints1.append( IVec(0, 0, 0) )
controlPoints1.append( IVec(20, 30, 30) )
controlPoints1.append( IVec(40, -30, -30) )
controlPoints1.append( IVec(60, 0, 0) )

deg1 = 3
ICurve(controlPoints1, deg1).clr(0)

controlPoints2 = []
controlPoints2.append( IVec4(0, 0, 0, 1) )
controlPoints2.append( IVec4(20, 30, 30, 0.5) )
controlPoints2.append( IVec4(40, -30, -30, 0.5) )
controlPoints2.append( IVec4(60, 0, 0, 1) )

deg2 = 3
ICurve(controlPoints2, deg2).clr(1.,0,0)

controlPoints3 = [ \
  [ IVec(-70,0,0), IVec(-70,20,30), IVec(-70,40,0) ], \
  [ IVec(-50,30,30), IVec(-50,50,60), IVec(-50,70,30) ], \
  [ IVec(-30,-30,-30), IVec(-30,-10,0), IVec(-30,10,-30) ], \
  [ IVec(-10,0,30), IVec(-10,20,60), IVec(-10,40,30) ] ]

udeg3 = 3
vdeg3 = 2
ISurface(controlPoints3, udeg3, vdeg3).clr(0)

controlPoints4 = [ \
  [ IVec4(-70,0,0,1), IVec4(-70,20,30,.5), IVec4(-70,40,0,1) ], \
  [ IVec4(-50,30,30,.5), IVec4(-50,50,60,.5), IVec4(-50,70,30,.5) ], \
  [ IVec4(-30,-30,-30,.5), IVec4(-30,-10,0,.5), IVec4(-30,10,-30,.5) ], \
  [ IVec4(-10,0,30,1), IVec4(-10,20,60,.5), IVec4(-10,40,30,1) ] ]

udeg4 = 3
vdeg4 = 2
ISurface(controlPoints4, udeg4, vdeg4).clr(1,.5,1)

To match the tangent of the edge of NURBS curves or surfaces, you can align control points at the edge and another adjacent one on a straight line.

add_library('igeo')

size( 480, 360, IG.GL )

curveEdgePt = IVec(10,0,0)
curveTangent = IVec(20,20,20)

cpts1 = []
cpts1.append( curveEdgePt )
cpts1.append( curveEdgePt.dup().sub(curveTangent) )
cpts1.append( IVec(0, 40, -40) )

cpts2 = []
cpts2.append( curveEdgePt )
cpts2.append( curveEdgePt.dup().add(curveTangent) )
cpts2.append( IVec(60,0,0) )
cpts2.append( IVec(100,0,0) )

ICurve(cpts1, 2).clr(1.,0,0)
ICurve(cpts2, 3).clr(.5,0,1)

# showing control points
for pt in cpts1 :
    IPoint(pt).clr(1.,0,0)

for pt in cpts2 :
    IPoint(pt).clr(.5,0,1)

# showing straight relationship
ICurve(cpts1[1], cpts2[1])

add_library('igeo')

size( 480, 360, IG.GL )

surfaceTangent1 = IVec(20, 0, -10)
surfaceTangent2 = IVec(20, -20, -10)
surfaceEdgePt1 = IVec(0, 0, 0)
surfaceEdgePt2 = IVec(0, 30, 0)

cpts3 = []
cpts3.append([])
cpts3[0].append(surfaceEdgePt1)
cpts3[0].append(surfaceEdgePt2)
cpts3.append([])
cpts3[1].append(surfaceEdgePt1.dup().sub(surfaceTangent1))
cpts3[1].append(surfaceEdgePt2.dup().sub(surfaceTangent2))
cpts3.append([])
cpts3[2].append(IVec(-10, 0, -30))
cpts3[2].append(IVec(-10, 30, -30))

cpts4 = []
cpts4.append([])
cpts4[0].append(surfaceEdgePt1)
cpts4[0].append(surfaceEdgePt2)
cpts4.append([])
cpts4[1].append(surfaceEdgePt1.dup().add(surfaceTangent1)) 
cpts4[1].append(surfaceEdgePt2.dup().add(surfaceTangent2))
cpts4.append([])
cpts4[2].append(IVec(10, 0, 30))
cpts4[2].append(IVec(10, 30, 30))

ISurface(cpts3, 2, 1).clr(0,0,1.)
ISurface(cpts4, 2, 1).clr(0,.5,1.)

# showing control points
for pts in cpts3 :
    for pt in pts :
        IPoint(pt).clr(0,0,1.)

for pts in cpts4 :
    for pt in pts :
        IPoint(pt).clr(0,.5,1.)

# showing straight relationship
ICurve(cpts3[1][0], cpts4[1][0])
ICurve(cpts3[1][1], cpts4[1][1])

To get a point on ICurve, you use pt() method with a double parameter u. Parameter u always ranges from 0.0 to 1.0.

add_library('igeo')

size( 480, 360, IG.GL )

cpts = [ IVec(-60,0,0), IVec(-20,0,0), IVec(20,0,60), IVec(60,0,-30) ]

curve = ICurve(cpts, 2)

pt1 = curve.pt(0.5)
pt2 = curve.pt(0.8)
pt3 = curve.pt(1.0)

# showing points
IPoint(pt1).clr(1.,0,0)
IPoint(pt2).clr(1.,.5,0)
IPoint(pt3).clr(0,.5,0)

# using them for creating other geometries
ICurve(IVec(0,60,-20), pt1).clr(1.,0,0)
ICurve(IVec(0,60,-20), pt2).clr(1.,.5,0)
ICurve(IVec(0,60,-20), pt3).clr(0,.5,0)

A tangent vector on ICurve can be calculated by tan() method with a parameter u.

add_library('igeo')

size( 480, 360, IG.GL )

cpts = [ IVec(-30,10,0), IVec(-10,0,0), IVec(10,0,30), IVec(30,0,-10) ]

curve = ICurve(cpts, 2)

tan1 = curve.tan(0)
tan2 = curve.tan(0.5)
tan3 = curve.tan(0.8)
tan4 = curve.tan(1.0)

# showing arrows at each point on the curve
tan1.show(curve.pt(0)).clr(1.,1.,0).size(10)
tan2.show(curve.pt(0.5)).clr(1.,0,0).size(10)
tan3.show(curve.pt(0.8)).clr(1,.5,0).size(10)
tan4.show(curve.pt(1.0)).clr(0,.5,0).size(10)

In the similar way to the ICurve, you can get a point on ISurface by pt() with u and v parameter. Parameter u and v both always range from 0.0 to 1.0.

add_library('igeo')

size( 480, 360, IG.GL )

cpts = [[ IVec(-40,-20,-20), IVec(0,0,0), IVec(40,0,-20) ], \
        [ IVec(-40,30,20), IVec(0,30,0), IVec(40,30,-10) ]]

surface = ISurface(cpts,1,2)

pt1 = surface.pt(0.5,0.5)
pt2 = surface.pt(0.5,1.0)
pt3 = surface.pt(0.3,0.2)

# showing points
IPoint(pt1).clr(1.,0,0)
IPoint(pt2).clr(1,.5,0)
IPoint(pt3).clr(0,.5,0)

# using them for creating other geometries
ICurve(IVec(-20,0,40), pt1).clr(1.,0,0)
ICurve(IVec(-20,0,40), pt2).clr(1,.5,0)
ICurve(IVec(-20,0,40), pt3).clr(0,.5,0)

You can get a tangent vector in u direction by utan() and in v direction by vtan().

add_library('igeo')

size( 480, 360, IG.GL )

cpts = [[ IVec(-40,-20,-20), IVec(0,0,0), IVec(40,0,-20) ], \
        [ IVec(-40,30,20), IVec(0,30,0), IVec(40,30,-10) ]]

surface = ISurface(cpts,1,2)

utan1 = surface.utan(0.5,0.5)
utan2 = surface.utan(0.5,1.0)
vtan1 = surface.vtan(0.5,0.5)
vtan2 = surface.vtan(0.5,1.0)

# showing arrows at each point on the surface
utan1.show(surface.pt(0.5,0.5)).clr(1.,0,0).size(5)
utan2.show(surface.pt(0.5,1.0)).clr(0,0,1.).size(5)
vtan1.show(surface.pt(0.5,0.5)).clr(1,.5,0).size(5)
vtan2.show(surface.pt(0.5,1.0)).clr(0,.5,1).size(5)

You can get a normal vector of ISurface by nrml(). There are also aliases of this method, which are nml() and normal().

add_library('igeo')

size( 480, 360, IG.GL )

cpts = [[ IVec(-40,30,20), IVec(0,30,0), IVec(40,30,-10) ], \
        [ IVec(-40,-20,-20), IVec(0,0,0), IVec(40,0,-20) ]]

surface = ISurface(cpts,1,2)

normal1 = surface.nml(0.5,0.5).len(40)
normal2 = surface.nml(1.0,0.5).len(40)
normal3 = surface.nml(0.0,0.5).len(40)
normal4 = surface.nml(0.2,0.8).len(40)

# showing arrows at each point on the surface
normal1.show(surface.pt(0.5,0.5)).clr(1.,0,0).size(10)
normal2.show(surface.pt(1.0,0.5)).clr(1.,.5,0).size(10)
normal3.show(surface.pt(0.0,0.5)).clr(0,.5,0).size(10)
normal4.show(surface.pt(0.2,0.8)).clr(0,.5,1).size(10)

You can get a point offset by normal with addition of pt() and nrml() like
surface.pt(0.5, 0.5).add( surface.nrml(0.5, 0.5).len(10) );
but there is also a method to get a point offset by normal, which is called also pt() but with three arguments of u, v and the offset length in the normal direction. The statement above can be written with this method like this.
surface.pt(0.5, 0.5, 10);

add_library('igeo')

size( 480, 360, IG.GL )

cpts = [[ IVec(-40,30,20), IVec(0,30,0), IVec(40,30,-10) ], \
        [ IVec(-40,-20,-20), IVec(0,0,0), IVec(40,0,-20) ]]

surface = ISurface(cpts,1,2)

pt1 = surface.pt(0.5, 0.5, 10)
pt2 = surface.pt(0.8, 0.5, 10)
pt3 = surface.pt(0.8, 0.8, 10)
pt4 = surface.pt(0.5, 0.8, 10)

# showing points
IPoint(pt1).clr(1.,0,0)
IPoint(pt2).clr(1.,.5,0)
IPoint(pt3).clr(0,.5,0)
IPoint(pt4).clr(0,.5,1)

# using them for creating other geometries
ISurface(pt1,pt2,pt3,pt4).clr(.5,0,1)