Inventor sketchspline to svg Bezier
I don't know of an easy way to accomplish what you want. The splines in Inventor are NURBS curves. NURBS introduces the additional concepts of weights and knots to a bezier curve. For a non-rational NURBS curve (which is the most common) I believe it should be possible to break up the NURBS curve into multiple bezier segments but it's not a trivial problem. If you have a way to fit a bezier curve through some points you might be able to approximate the curve by fitting a series of bezier curves through points on the NURBS curve.
The easiest thing will be to approximate the curve with a polyine. You can determine the tolerance for this approximation so it can be very close to the original spline. To do this, from the SketchSpline you can use the Geometry or Geometry3d properties. The Geometry property will return a BSplineCurve2d that represent the spline in sketch space and the BSplineCurve3d property will return a BSplineCurve object that represents the spline in 3D model space. From the BSplineCurve or BSplineCurve2d object you can call the Evaluator property which will return CurveEvaluator or CurveEvaluator2d object. These object support the GetStrokes method. This will approximate the spline by a series of points where the result is within a given tolerance.
I thought that if I broke up a curve along the knots so I ended up with multiple curves with no internal knots that would be easy to convert to a Bezier equivalent. A problem is that it appears that you're limited to cubic Bezier curves. In my few tests, Inventor is creating curves with an order of 5. To go to four will require an approximation. The math could be interesting but it's not trivial and would take some time to work out. There are quite a few books on curve math and also some programming libraries. Good luck.
You definitely can't ignore the first and last control points because those are the first and last points of the curve.
I did some playing around before I responded last time. Here's the code if it helps at all. It breaks the curve up at the knot locations along the curve so that the result is a series of curves without any internal knots. Assuming the original curve is not rational, these curves are essentially bezier curves but Inventor is creating order 5 curves and you need order 4. I'm not sure how to do that convesion and believe it will be an approximation.
This code works the best if the sketch containing the curve was created on the base x-y work plane. It creates a new sketch and draws the result and that way the two will line up.
Public Sub BreakCurve()
Dim skSpline As Object
Set skSpline = ThisApplication.CommandManager.Pick(kSketchCurveSplineFilter, "Select a spline")
Dim sk As PlanarSketch
Set sk = skSpline.Parent
Dim spline As BSplineCurve2d
Set spline = skSpline.Geometry
Dim order As Long
Dim numPoles As Long
Dim numKnots As Long
Dim isRational As Boolean
Dim isPeriodic As Boolean
Dim isClosed As Boolean
Call spline.GetBSplineInfo(order, numPoles, numKnots, isRational, isPeriodic, isClosed)
Dim poles() As Double
Dim knots() As Double
Dim weights() As Double
Call spline.GetBSplineData(poles, knots, weights)
Dim eval As Curve2dEvaluator
Set eval = spline.Evaluator
Dim min As Double
Dim max As Double
Call eval.GetParamExtents(min, max)
Dim knotList() As Double
ReDim knotList(numKnots - (order * 2 - 2) - 1)
Dim i As Integer
Dim cnt As Integer
cnt = 0
For i = order - 1 To numKnots - order
knotList(cnt) = knots(i)
cnt = cnt + 1
Next
Dim newSketch As PlanarSketch
Dim partDoc As PartDocument
Set partDoc = ThisApplication.ActiveDocument
Set newSketch = partDoc.ComponentDefinition.Sketches.Add(partDoc.ComponentDefinition.WorkPlanes.Item(3))
For i = 0 To UBound(knotList) - 1
Dim newSpline As BSplineCurve2d
Set newSpline = spline.ExtractPartial(knotList(i), knotList(i + 1))
Call newSketch.SketchFixedSplines.Add(newSpline)
Next
End Sub
The splines in Inventor are apparently all order 5 and you need order 4 for a quadric. I believe to reduce the order of the curve it will mean creating an approximation. Increasing the order can be done without losing precision, but reducing is an approximation.
Arc fitting isn't as easy as it first appears because ideally you're fitting to a certain tolerance to minimize the number of arcs and you also need to worry about tangency between the arcs.
Splitting the curves into smaller segments won't reduce the order. Inventor will just keep creating order 5 curves. There's nothing in Inventor that will allow you to convert an existing curve into a new curve with a different order.
