The new 3D modeling tools in AutoCAD 2011 required some updates to the spline command, and while a lot of them are most applicable to 3D situations, some of them have trickled over to the 2D side.
You’ll notice the first change right after you start the command, if you pay attention to the options at the dynamic prompt or command line.
“Object” has been around for a while (it turns a polyline into a spline), but “Method” and “Knots” are new to AutoCAD LT 2011.
“Method” lets you choose between “Fit Points” and “Control Vertices.” Fit Points is the old familiar way, where the spline passes through each point that you select. With Control Vertices (CV), you essentially define bounding lines and the spline fits between them. (That’s an overly simplistic explanation, but let’s just say that CV splines are important for 3D modeling, and you should go check out Guillermo Melantoni’s What A Mesh blog if you want to know more.)
Here are examples of the two methods, first with fit points, then with CV.
Whichever method you choose, you can change the display later.
Knots affect the shape of a fit spline as it passes through fit points. There are 3 options (Chord, Square Root, and Uniform), but I’m honestly not sure how to explain the differences.
Maybe the biggest change you’ll notice is that when drawing a fit spline, the start and end tangencies are now assumed. You can change it with the Tangency options if you want to, but you don’t have to. (Admit it – when you finish a pre-2011 spline, you just hit Enter-Enter-Enter anyway, right?)
Splines also have some new fancy grips, and they’re multifunctional like the ones for polylines and hatches.
The options you’ll see are a little different depending on whether you’re working with a fit spline or a CV one, but the concept is the same.
In the image above, the triangular grip is for switching between Fit and CV display. The “plus” on the left-hand grip indicates the beginning of the spline (to help identify “start” and “end”). For the square grips, you can stretch, add, and remove fit points, and for the start/end grips you can change the tangent direction.