The documentation on draw_spline says:
"A cubic Bezier curve is sometimes defined by a set of 4 points { (x0,y0), (xa,ya), (xb,yb), (x1,y1) } where (x0,y0) is the starting point, (x1,y1) is the ending point and (xa,ya), (xb,yb) are two control points. The starting and ending velocities (u0,v0) and (u1,v1) can be deduced easily from the control points as u0 = (xa - x0), v0 = (ya - y0), u1 = (x1 - xb) and v1 = (y1 - yb)."
This is wrong - the differences of the coordinates need to be multiplied by 3 ->
"A cubic Bezier curve is sometimes defined by a set of 4 points { (x0,y0), (xa,ya), (xb,yb), (x1,y1) } where (x0,y0) is the starting point, (x1,y1) is the ending point and (xa,ya), (xb,yb) are two control points. The starting and ending velocities (u0,v0) and (u1,v1) can be deduced easily from the control points as u0 = 3(xa - x0), v0 = 3(ya - y0), u1 = 3(x1 - xb) and v1 = 3(y1 - yb)."
The documentation on draw_spline says: "A cubic Bezier curve is sometimes defined by a set of 4 points { (x0,y0), (xa,ya), (xb,yb), (x1,y1) } where (x0,y0) is the starting point, (x1,y1) is the ending point and (xa,ya), (xb,yb) are two control points. The starting and ending velocities (u0,v0) and (u1,v1) can be deduced easily from the control points as u0 = (xa - x0), v0 = (ya - y0), u1 = (x1 - xb) and v1 = (y1 - yb)."
This is wrong - the differences of the coordinates need to be multiplied by 3 ->
"A cubic Bezier curve is sometimes defined by a set of 4 points { (x0,y0), (xa,ya), (xb,yb), (x1,y1) } where (x0,y0) is the starting point, (x1,y1) is the ending point and (xa,ya), (xb,yb) are two control points. The starting and ending velocities (u0,v0) and (u1,v1) can be deduced easily from the control points as u0 = 3(xa - x0), v0 = 3(ya - y0), u1 = 3(x1 - xb) and v1 = 3(y1 - yb)."