curvature comb T-Splines vs Brep

[cekuhnen]

Hi folks

Is if my impression or is there a significant difference between a curvature comb of a brep or spline curve compared to a t-spline edge loop or mesh surface?

I have the feeling that the t spline result is not as smooth and accurate showering many kinks in the comb.

Claas

[deyop]

Although T-Splines have topology and can be manipulated in a manner similar to polygonal models they also have the ability to convert to NURBS geometry.  Each face of the T-Spline can converted to a 3rd degree Bezier surface.  When we visualize the model or apply curvature combs to the T-Spline we are actually evaluating the 3rd degree Bezier surface.  This is the reason the curvature combs are displaying differently than splines which are 5th degree.

The display is accurate however the relative smoothness is a point of argument.  The question becomes whether it is smooth enough for the purpose it will be used either for visualization or manufacturing.  The curvature comb emphasizes the relative smoothness but the practical application of the results is what is important.

We are planning on changes in the future to ensure curvature continuity at the star points.  We have also discussed raising the degree of the T-Spline to 5th degree however there would be perfomance and interaction comprimises that may not justify making the change.

We certainly are interested in your comments.

Thanks

[cekuhnen]

Hey Deyop,

you raised some good points about the feasibility of a more precise representation. Spline combs are perfect while the Brep comb is very similar to the T-Spline comb including the kinks.

Maybe coming from Alias I am very used to work with the combs and trust them. The moment I noticed some kinks my initial reaction was asking myself if the surface has issues.

But reading between the lines it seems that the kinks are more a problem of generating the comb with a T-spline body while the underlying geometry is actually smooth to perfection. Right?

[deyop]

I think you can continue to trust the combs but consider if you were to make a degree 3 curve in Alias.  The comb will look similar to the T-Spline comb.  When you need additional flexibility you will have to add edit points to the curve.  At those points you will see the same type of curvature "kinks".  Is the T-Spline smooth?  I won't try to persuade you only let you know that degree 3 surfaces have been used successfully in manufacturing for years.   From the long time Alias user perspective it may be suspect though.

When it comes to the converted BRep surfaces they also are degree 3 so they will show the same curvature comb characteristics.

Thanks

[cekuhnen]

I don't not trust the usability of T-Splines for surfacing. And Alias being used for transportation design also puts a different focus on surface perfection simply because of the transportation design aesthetic and requirements.

If you look at many consumer products you can spot many G1 edge rounding which drive me mad as a designer.

I think at the end it comes also down to being skilled at sculpting. I don't have the impression that using T-Splines will produce a bad surface.

I was just curious about the technicality behind the comb display.