Unexpected Curve.CreateBlendCurve result in RhinoCommon (C#)

Rhino Developer
1

image

In the image above, all curves are 2D planar curves. The red curve is the desired result. It was produced using a default application of the interactive command BlendCrv. In C#, all my efforts to duplicate the red curve turn out looking like the blue curve. I’ve tried everything I can think of to change the result and it just keeps returning the same result every time. Here’s the code snippet I’m using:

rhCrvBlend = Curve.CreateBlendCurve(rhCrvParent, rhCrvParent, BlendContinuity.Tangency);
doc.Objects.AddCurve(rhCrvBlend);

I’ve tried all sorts of variations on the theme using a single parent curve and multiple parent curves, and I get the same result in Rhino 5 32-bit, Rhino 5 64-bit, and Rhino 6. I’ve tried changing the BlendContinuity to Curvature and it only makes the sharp corner less conspicuous. I don’t understand why RhinoCommon is “throwing” the control points so far away from the parent curves. When I interactively construct the BlendCrv, the control points do not extend past the midpoint of the created curve. I’ve tried scaling the model by a factor of 100 and I get the same results, so it’s not a matter of tolerances or some weird snap setting (I tried disabling Osnap, Ortho, GridSnap, and SmartTrack in code, too, just in case).

2

You’ll need to set the „bulge“ factor as well. The cps should be equally spaced as default factor 1.0, but this is not how this particular algorithm works. A clear weakness of this native method.

3

Well, that’s disappointing. Now I need to figure out what the black magic is inside the black box of BlendCrv so I can specify the “bulge factors” in code, or learn how to activate the BlendCrv command from C#.

Better documentation is needed here, I think. :frowning:

4

@davidjgall,

if tangency is enough you might use CreateInterpolatedCurve as a workaround and provide the tangent vectors. It is less work to find the proper tangents and if you need to flip one.

_
c.

5

Thanks, that’s exactly the workaround I found. Since I’m working with both ends of a closed curve, I had to flip both end tangents. This gave me exactly the same result that BlendCrv gave. Problem solved.