topic Re: [BUG] T-Spline cannot be fixed using Utilities/Make Uniform in Fusion Support Forum

[BUG] T-Spline cannot be fixed using Utilities/Make Uniform

TrippyLighting — Sun, 17 Apr 2022 22:45:46 GMT

This is a very old bug and it's time getting to the bottom of it.

Usually I only see it in conjunction with creases or more precisely when user continue to model with creased edges.

However, this model exhibits the problem and has never seen a crease, as I avoid creases in my models.

I already needed to use "repair body" to eliminate duplicate vertices (red start points) which should not have been created to begin with.

The smooth view topology shows terrible artifacts, which, in theory, should be fixed by "Make Uniform", but that doesn't do anything. I had to export the control cage as a .obj, re-import the mesh an convert it into a T-Spline. Then it looks ok.

Original:

Re-imported:

Re: [BUG] T-Spline cannot be fixed using Utilities/Make Uniform

adam.helps — Tue, 19 Apr 2022 23:45:52 GMT

I agree that we need to do something about this.

The root problem is usually either creases or T-junctions. The crease problem is a kind of UI problem; T-Splines can crease individual vertices, but Fusion creasing is edges only. I'm not totally clear on how to get into that state, but if you get a lone creased vertex, the workaround is to crease a nearby edge and then uncrease it, which removes the vertex crease at the same time. It's not at all intuitive.

The other problem is usually T-junction related. Utilities > Repair Body is able to edit these. Again, though, that's not something a lot of users know how to do.

When you export/import an OBJ, you are effectively deleting all creases and turning T-junctions into star points, because OBJ doesn't support either thing. Essentially, it turns the T-Spline into an uncreased subd. It might be a good idea to just make a "Convert to Subd" command, because that's pretty much what it's doing. A first attempt might be just as ham-fisted as the OBJ import/export version, though...

Re: [BUG] T-Spline cannot be fixed using Utilities/Make Uniform

TrippyLighting — Wed, 20 Apr 2022 01:08:36 GMT

@adam.helps thanks for chiming in!

The thing is that in this particular case I did not use any creases!

What I remember from previous exchanges is that creasing is the result of manipulating T-Spline tangent handles.

"Make uniform" is supposed to "normalize" the tangent handles, but often that does not seem to work.

Exporting the quad control cage to .obj then deletes the tangent handles because .obj and sub-D meshes don't have tangent handles. Those will then be calculated from scratch when the .obj mesh is imported.

It might be heavy handed, but a "re-compute tangent" handles or whatever you want to call the function, would certainly help a lot of users who have this problem. Few people know the method of exporting to .obj and re-importing, and obviously that only works if you stick to a quad mesh topology.

Re: [BUG] T-Spline cannot be fixed using Utilities/Make Uniform

glenn-chun — Fri, 13 May 2022 18:54:30 GMT

Hi Peter,

You can fix your original T-Spline surface by converting its four T-points to star points, using the Repair Body tool. See the attached animation.

Glenn

Re: [BUG] T-Spline cannot be fixed using Utilities/Make Uniform

TrippyLighting — Fri, 13 May 2022 19:15:11 GMT

@glenn-chun Thanks for the demo. I was not aware that could be done.

The documentation on the repair panel is sparse at best. That little video you made along with a short paragraph would be a very welcome addition to that page and would be incredibly helpful!

The question I still have is why those points tuned into T-Points ?

I usually model strictly in quad geometry.

Re: [BUG] T-Spline cannot be fixed using Utilities/Make Uniform

adam.helps — Fri, 13 May 2022 19:44:06 GMT

"The question I still have is why those points tuned into T-Points?"

I also have this question, because it's probably something that would be good to fix. A T-point on the side of a triangle is usually a bad idea. Obviously it's possible, but how did we get there? Unfortunately, without a recording of the modeling process, I've just got to guess.

I did try this:

Deleting edges to combine faces often results in a T-junction. In this case, though, I didn't end up with a T-junction, I ended up with a quad, which is the better answer. I guess there's some sequence of operations where deleting/splitting results in the T-junction/triangle combo. If we can find it, that would be a good first step toward fixing it. Keep an eye out, I guess?