Spline degree, types, curvature and surfaces

[Intuos5]

Lately, I have been reading into the different type of splines offered by Fusion and the underlying geometric principles. From my short experience with both, it seems that control point splines are much easier to use in 3D space as I am manipulating points, which unlike control handles do not inherently move in two directions after manipulation. I have also read Sketch control point splines FAQ, which suggests that control point splines serve better accuracy in curvature continuity with the option to move control points to manually crease edges.  This got me wondering, other than ease of use, what other reasons are there to use fit point splines in both 2D and 3D? Are there other possibilities to constrain such sketches that only work in one type of spline and not in the other, etc?

I was also wondering: if curvature continuity is the only thing perceivable by eye from a surface, what is the reason to go beyond that? And what use-cases require degree 5 precision over 3? And why is there even such a thing as degree 7 in other software? My best guess would be that it has something to do with the manufacturing process...

[TrippyLighting · Accepted solution]

CV splines (control vertex splines) are either 3 degree or 5 degree splines, depending on what you choose.

Fit point splines are always 5 degree multi span splines.

Both have they uses.

Both can be used to create surfaces in Fusion 360, however the geometric kernel in Fusion 360 (and Autodesk Inventor) can only create 3 degree surfaces. I could try to explain degrees, but the Autodesk Alias Theory Builder does a much better job that I can do,

[Intuos5]

Thanks, that's a good read!

So, if certain youtube tutorials state that they resort to other packages for precise surface modeling, this has to do with G3/ G4 continuity, am I right? And limitations to t-spline technology as degree 3 surfaces are not so much of an issue as that's as far as Fusion goes anyways, since it's also limited to degree 3 surfaces? I just want to make sure that I understand all of this correctly. 

[TrippyLighting]

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@Intuos5 wrote:

Thanks, that's a good read!

So, if certain youtube tutorials state that they resort to other packages for precise surface modeling, this has to do with G3/ G4 continuity, am I right? 

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Can you provide. link to these tutorials ? They might have a point!

On the other hand a lot of people while well intended make uninformed tutorials and make false statements.

For all the most sophisticated product designs G3 is really not necessary. It is mislay used in vehicle design where large glossy surfaces dominate. Applying that correctly is bot only a matter of having the right tis available but more so having an understand of what and how to apply the tools.

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@Intuos5 wrote:

And limitations to t-spline technology as degree 3 surfaces are not so much of an issue as that's as far as Fusion goes anyways, since it's also limited to degree 3 surfaces? 

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Trying to theorize about limitations without practical examples where those apply is really pretty useless.

What are the questions you are trying to answer ?

[Intuos5]

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@TrippyLighting wrote:
Can you provide. link to these tutorials ? They might have a point!

On the other hand a lot of people while well intended make uninformed tutorials and make false statements.

For all the most sophisticated product designs G3 is really not necessary. It is mislay used in vehicle design where large glossy surfaces dominate. Applying that correctly is bot only a matter of having the right tis available but more so having an understand of what and how to apply the tools.

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Well it's mostly this statement that I can recall (t: 5 min 27s): 

Though precision may also be key, the Alias theory builder - Curvature degree  answered my question about curvature degrees (see diagrams with dimensions). Thanks for the tips, I am starting to get the point!

[TrippyLighting]

I've seen that video before and it is one of my most favorite Fusion 370 tutorials!

He's made a few more and you should watch all of them as at least one of them highlights a few problem areas in Fusion 360.

His point point about the star point interfering with surface quality is correct but he also mentions that it can be removed but at the expense of ease of control. That means the mesh flow has to be changed and geometry added. the more geometry you add the more polygons you have to control.

[Intuos5]

I have watched all of his tutorials twice by now, except for the shoe modeling as that is something I plan to follow along at some point. Concerning mesh flow, I know there are tons of tutorials covering this topic for 3ds Max/ Maya, so that's something I'll be looking into further at some point. 😉 I wish there were more tutorials that cover advanced techniques like this guy does though. Perhaps there are a few for Alias Speedform, I've got some on my radar as well. Got lots of work to do I guess...!

[TrippyLighting]

Unless you want to get into Alias Studio I would not invest in Alias Speedform and an investment it is. Both in financial terms and in terms of a rather step learning curve.

The lowest subscription level for Alias (Concept) is $530 a month.

Speedfrom uses the same T-Spline technology available in Fusion 360, perhaps with a few more tools. It's main selling point AFAIK is that it integrates into Alias Studio.

[Intuos5]

I may have not been clear enough, I meant it in terms of t-spline modeling tutorials, since both packages are so similar. Plus users of Alias seem to be more skilled at surface creation (as that's what the software is for after all) unlike the many contradictory and sometimes faulty tutorials found on youtube. 

On a side note: Speedform does have some interesting features such as: Photoshop live link for canvases, Dynamo integration, and a couple of additional t-spline modeling tools, such as:  cage (lattice) deformation and point to point pivot adjustments. And apparently also this: "the ability to freeze parts of the T-spline surface to "lock them down" when they have hit the correct shape, enhanced ways to manipulate cvs, etc." - Matt Sederberg 
Perhaps some of these may be of interest for Fusion nonetheless. Though Autodesk may want to artificially differentiate between the two.  

[TrippyLighting]

Agreed. The Fusion 360 level of T-Spline tutorials rarely explains their true potential or proper techniques. I often refer users to the old T-Spline tutorials for Rhino. The reason for that is likely that a lot of the people that do Industrial Design in Alias know how to work with Polygon Modelers such as Modo or Blender.

I mostly design my more complex T-Splines in Blender because the tool set is much more complete and capable then the one in Fusion 360. That way I can take advantage of the speed of a polygon modeler and the T-Spline specific tools in Fusion 360.

The freeze function BTW is also available in Fusion 360.

I also believe that certain functions are reserved to Speedform.