T Parameter Defintion Alon a Curve in RhinoCommon

What is the definition of the t length parameter used to define a position laong the length of a curve. I had thought it was simply a normalized value from 0 to 1 representing a percentage distance along a curve but from the results of my code that is not true and one must also frist determine the value of t i.e.:

To get a point at the mid point of a curve:

Dim rcTParameter As Double

ghobject.Value.LengthParameter(ghobject.Value.GetLength * 0.5, rcTParameter)

rmaPoint.X = ghobject.Value.PointAt(rcTParameter).X

I had hoped that t was betwwen 0 and 1 and I could have simply written:

rmaPoint.X = ghobject.Value.PointAt(0.5).X

thanks

Replies to This Discussion

Permalink Reply by David Rutten on September 12, 2010 at 4:42pm

Hi Duck,

Curve.PointAt() uses curve parameter space which may not be (indeed most often is not) linear. Therefore the halfway point on the curve domain is only rarely also the halfway point on the curve.

You could use the Curve.NormalizedLengthParameter, which works in the 0.0~1.0 range:

Dim t As Double

If (Not ghobject.Value.NormalizedLengthParameter(0.5, t)) Then Return

rmaPoint.X = ghobject.Value.PointAt(t).X

It would be easier in some cases if NormalizedLengthParameter returned the double instead of a boolean, but at least this way it is obvious that the function can fail and that you should always check the return value for validity.

I do see your point though, I'll have a pow-wow with Steve to see if we're happy adding a special PointAt() method that takes normalized length parameters instead of curve parameters.

--
David Rutten
david@mcneel.com
Poprad, Slovakia

Permalink Reply by Steve Lewis on September 12, 2010 at 4:48pm