# T-Splines / Imagine & Shape

I’m a bit unsure how these work. Am I right in saying that T-Splines uses Sub-D modeling and is not actually NURBS. So therefore it is polygon data which can be converted to NURBS later?

http://www.tsplines.com/resources/rhino/T-SplinesInClassroom.pdf

Then how about Catia’s Imagine and Shape. Does this use Sub-D modeling and NURBS surfaces?

Hi, I’m a developer for T-Splines. Saw this come up on my alerts, figured I’d give a quick response…

Sub-D modeling is used in Maya, T-Splines, and Imagine and Shape, and they all need to keep a control mesh around to work on. They all convert to NURBS when requested. The conversion can’t be exact near star points (extraordinary points), for mathematical reasons.

T-Splines for Rhino converts to NURBS for display, so anytime you’re looking at a T-Spline, you’re actually looking at the NURBS conversion (you can toggle this option for speed if you want). I imagine that Imagine and Shape is doing something similar, but I honestly don’t know. Maya only converts when you run the convert command, and most of the time is successful, although there are a few unusual cases where the conversion fails.

The main difference with T-Splines is that the surface type is compatible with degree-3 NURBS - that is, you can convert any untrimmed degree-3 NURBS to a T-Spline, and it remains exactly the same shape, with the same control points in the same positions. Then you can take that shape and do Sub-D style modeling with it, extruding faces, etc. You can also have rows of control points that T-off, stopping abruptly while keeping a well-behaved smooth surface. So, you’ve got more options for how to lay out your topology. In terms of NURBS compatibility, any command that takes NURBS surfaces in Rhino will take T-Splines as well.

I think Imagine and Shape’s main advantage is that they deal well with trimmed surfaces, and they’re integrated with Catia. But I’ve never been able to try Imagine and Shape, so I don’t really know.

Again, I’m obviously biased, but this is my understanding of how the surfacing technology of the three products compare. Hope it’s helpful…

Thanks for the info Tom. So in essence you’re working on a polygon cage which is being converted live to Nurbs (maybe it goes from polygons to a sub-d to a Nurbs conversion?).

Do you get exactly the same result with a surface contaning a T-Off compared to one that doesn’t?