what degrees do splines in Fusion have?

[cekuhnen]

Are they G3 G5 ...???

When using Tangent constraint or smooth constraint will the degree adjust?

Or how does this work in Fusion with Splines?

I know Inventor has Fit point and CV splines.

[deyop · Accepted solution]

Splines in Fusion Sketch are 5th degree.  Degree 5 curves ensure the ability to create a curvature constraint (Smooth) on both ends of a single curve.  Degree 5 curves also provide rate of curvature continuity at interior edit points. Utilizing 3rd degree curves cannot ensure the same level of curvature continuity.  

Whether you control the shape of the curve with edit/fit/Sketch points or control vertices the underlying characteristics of the curve are the same depending on what degree the curve is.  Fusion hasn't yet exposed control vertex editing but the curves in Sketch are degree 5.

I will try to review some of the curve concept.  Keep in mind I am not a developer or mathematician.  This is my insight based how I have seen curves utilized in design.  Based on your other posts I will expand the conversation a bit to give some background.  When Bezier curves are referred to it means they are of a single span and each span is perfectly smooth.  When these pieces of curve are connected together in a single curve or multispan curve they are connected at edit points.  There is often controversy among digital modelers about the benefits of using single span Bezier or multispan NURBS (NonUniform Rational B-Splines).  The controversy usually centers around two issues, the smoothness of the curve and the ability to manage the complexity of the segments.  There is little data to support the superiority of Bezier vs. NURBS but there is certainly a lot of opinions. 

If you look at the illustration below I have shown single span and multispan instances of different degree curves.  Degree 1 is linear, point to point or polyline.  Degree 2 is the most simple curve shape which can never create an unintended inflection point in a single segment.  The conic curves we provide are degree 2 and are useful because they can always ensure a transition that will not inflect.  Degree 3 curves have enough flexibility to provide an inflection point when needed.  However a single span degree 3 curve does not have enough control to provide curvature continuity at both ends.  It can only provide tangent continuity as a transition curve.  You have pointed out in the past that the curvature comb does not look as smooth as degree 5.  It is only curvature continuous or G2 at the edit point between spans.  Looking at the degree 5 curve you will see that the connection at the edit point is better than curvature.  It is rate of curvature continuous or G3.

There is an example of three single span degree 5 curves.  They illustrate that the single degree 5 curve has enough control to ensure curvature continuity at each transition.  By control I mean that there is vertex to maintain position, a vertex to ensure tangency and a vertex that can manage the curvature continuity.  The constraints we provide for tangency and smooth are utilizing this control.

So finally in answer to your question the splines in Fusion are degree 5 and can ensure curvature continuity at both ends without adjusting the degree and will be curvature continuous at any Sketch Point along the spline.

Thanks.  I hope this was helpful.

[cekuhnen]

Question when you add a fit point is that equal like in Alias having a span?

I guess multi span are important for managing the sculpt-able amount of CVs and providing light weight data for ICEM and such when prepping it for manufacturing. if you need to make changes complex surfaces can be a problem to work with. I guess thats why some tools have the ability to create single span multi surface results in Alias. But then that blows up the model with many patches.

[cekuhnen]

that also would mean no G3 blends when D5 is the max?

[jiang.peng]

I am not 100% sure - I didn't get the chance to look deeply into Alias spline. But I guess the answer is no: adding a fit point in Fusion is not like adding a span in Alias. It looks to me the "Edit Points" of Alias are the points at the knot vector values. In fact we are using the same technology of spline which Inventor is using. One example is that if you create a closed spline which fit points are at 4 corners of a cube, you will get a spline that looks like a circle(but not accurate). Again, I am probably wrong.